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PapaMidnite

Blues Custom Mods

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I finally got around to modding my BC30 last night. I cut C3, C5, and C6, and also changed out R15 and R21. What I was lacking was the electrolytic cap needed to change C15, and also realized the board will have be removed to replace it, so I skipped C15, but I would like to know what effect dropping the value from 22uF to 2.2uF has.

 

The results are largely positive, the amp sounds looser, with a little more sag it seems, and fuller. The drive and interactive channels are vastly improved, offering a lot of tonal variations, and the tone stack functions within the normal ranges with those channels. I was getting some low end thump from the interactive channel that was very impressive.

 

The issue is with the clean channel, which is quite dark compared to the drive channel. I'm having to turn the treble up to 9 and the bass down two 2 to add any bite to the tone. The tone is also fuller like the other channels, but I'd like to figure out a tweak to balance the clean channel out. As is, it will be impossible to switch between channels without adjusting the tone controls. I wonder if I could reconnect one of the caps using a lower value and achieve my goal--anyone have an idea on how to?

 

As of now there are stock power tubes and speakers in my BC30, with Tung-sol reissue preamp tubes. I bought a set of Tung-sol reissue 5881 tubes to go in it, and I may try it with the Red Fang speakers in my Black Pearl. The sound of the wound strings through the Lady Lucks is pretty good, but the top end sounds flat to me, so new speakers are a definite must.

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Last night I experimented some more. I diverted the output of the BC30 into a pair of 16ohm Eminence Red fang speakers, and the sound was very good, with pronounced low end, but still somewhat on the dark side, meaning the clean channel.

 

Next I swapped out the stock power tubes with a set of Tung-sol 5881 reissues, and the difference was dramatic, it was like lifting a blanket off the amp. Now it has Tung-sol reissue power and preamp tubes, and a JJ rectifier tube. Through the red fangs the top end was sweet and the low end well-defined, but I don't think the red fangs are the exact match I'm looking for, and note that most amps will sound decent through $350 worth of speakers. What the BC30 needs are some good American sounding speakers, the red fang has an over the top British tone.

 

I hooked the Lady luck speakers back up and the sound is good, the clean channel is not as dark as it was, so it may be alright as is. The next choice is a better set of speakers, but I may change out C15 if I can find the right cap, but I'd still like to know what what the change brings.

 

EDIT:

 

I played on the BC30 some this morning and I'm impressed with the way it sounds as is. With the treble at 7, the mid at 5, and the bass set at 4, the clean channel sounds beautiful. The change in tubes fixed the extreme darkness in the clean channel.

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Good stuff guitslinger! So, how's channel 2 sounding?

 

I'm quite happy with how channel 1 sounds on my amp. I now would like to do something about channel2. The drive is just "harsh" is the best way I can describe it. It's much better than it used to be but it still needs to be tamed. I'll give the 5881s a try as your description sounds intriguing.

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Good stuff guitslinger! So' date=' how's channel 2 sounding?

 

I'm quite happy with how channel 1 sounds on my amp. I now would like to do something about channel2. The drive is just "harsh" is the best way I can describe it. It's much better than it used to be but it still needs to be tamed. I'll give the 5881s a try as your description sounds intriguing.[/quote']

 

 

Harshness is typical of most tube amps that are in need of new tubes no matter what the cost or design of the amp, so the first step in dealing with harshness is having a set of spare tubes waiting.

 

I played all of the channels for awhile tonight, and even though harshness can be introduced by turning the treble control up too much, I can't say the amp sounds harsh at all given some reasonable tone tweaks. The good thing is I no longer have to turn any of the tone controls to an extreme to get acceptable tones, the bad thing is the channels are voiced quite differently and require adjustments to the tone controls when changing channels. The clean channel requires turning the treble up to around 7 and the bass down to 4, while the drive channel is the inverse being brighter, the treble was set at 4 and the bass at 7, more or less. The mid control can introduce harshness with any amp, so it must be used judiciously.

 

I will agree the drive channel is a little thin sounding, but adding bass compensates adequately to get useful sounds, but it would be nice to fatten the overall sound of the amp. One option is speakers, and another is a different output transformer, but I'm not sure I'm willing to invest a lot more in my BC30. As is, it makes a good backup amp.

 

I should also add that my tests were conducted at bedroom volume levels: I'm sure that cranking the volume on my BC30 would fatten and improve the overall sound, as is the case with most tube amps.

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Guitslinger et al,

 

I found that it is possible to exchange caps and resistors from the top of the board if you can flow a little tin/lead solder down the new component legs, the capillary action flows it onto the board, but beware the original solder can explode away from the legs, fortunately the two types are mixable.

 

Regarding C15

Cathode decoupling caps are chosen to give to give a bottom roll off frequency to the valve section. A large cap value will give a lower frequency than a smaller one. I have seen values of 220µF used to get 1Hz through whereas 22µF would only pass 10Hz. [ref ISBN 978-0-7506-5694-8]

 

The drive channel uses V1B and V2A. V2A has no decoupling cap. When I added a 22µF across its R15 I got a lot more bass. The cathode decoupling cap will also increase the voltage on the cathode, which affects its gain. I used values of 2.2µF, 4.7µF, 22µF, 47µF but could not hear much difference and did not like what I heard, so I removed them again. However, putting a 22µF cap in and reducing the gain may well work, perhaps with a resistor in series like the 'fat' switch on the Blues Junior, or the gain pad on the Laney L5T. I do not use my bright switch so a fat switch is a likely next-mod candidate.

 

I have changed R15 to 3.2k by putting a 4.7k shunt across the 10k in place. That gave the drive channel more bass.

 

The cathode resistor sets its voltage.

The cathode decoupling cap determines the lowest frequency

The anode resistor determine the gain.

 

Do not monkey with the DC decoupling caps C2, C9, C12

 

There is an odd thing going on here. If one uses half a double triode, you should use the best 'B' section, and if you have an extra gain section, you normally use a separate valve for it. So V1B should have been be V2A, V2A should have been V2B and V2B should have been V1B. It is not possible to re-wire the PCB ;-(

 

Changing C15 from 22µF to 2.2µF should make the whole amp brighter, less bottom end. It may be more effective to alter its cathode resistor R21, mine is 1.5k not the 2.2k shown on the ver1 schematic.

 

Modding C16 to 680pF gave me less shrill treble. I don't know if it is the Greenbacks but if I dare crank the bass round to 3 o'clock I get a huge Marshallesque bass roar, I normally operate the tone controls below 12 o'clock.

 

Incidently I lifted (by de-soldering) one leg of C3 and C5 up but put C4 and C6 on a DPDT bright switch but note the values were changed from stock to schematic (was my board mis-assembled?)

Values now -

C3=C5=471 one leg lifted

C4=C6=101 on bright switch (always off for me)

These caps have been in and out so many times that I have fitted Harwin pins for them.

 

The master volume mod does affect the tone in a good way too, and if I had to pick the single most important mod, this would be it.

 

A bit muddled but hope it helps

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Guitslinger et al' date='

 

I found that it is possible to exchange caps and resistors from the top of the board if you [i']can[/i] flow a little tin/lead solder down the new component legs, the capillary action flows it onto the board, but beware the original solder can explode away from the legs, fortunately the two types are mixable.

 

Regarding C15

Cathode decoupling caps are chosen to give to give a bottom roll off frequency to the valve section. A large cap value will give a lower frequency than a smaller one. I have seen values of 220µF used to get 1Hz through whereas 22µF would only pass 10Hz. [ref ISBN 978-0-7506-5694-8]

 

 

Thanks for the info on C15, which makes sense, larger cap values create more bass frequencies, and in that case, I'm glad I chose not to change it.

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More thoughts on the BC30.

 

All of my experiments conducted over the weekend were done with the back off, and it occurred to me that with the back off much of the fullness and bass frequencies that happen with a closed back combo were being lost. I put the back on and tested all three of the channels tonight, and went away feeling good about the BC30.

 

The clean channel sounds good with no extreme settings to the eq section: plenty of bass, plenty of clarity, with no harshness. Switching from the clean channel to interactive mode sounds good, plenty of good overdriven sounds there. What I'm learning is that even with the changes made to the circuitry, the tone controls still act differently than one would expect--there are a lot of hidden tones in the BC30 that can be coaxed out by twisting the eq knobs every which way.

 

The issues with the drive channel being overly bright remained until I cranked the mid control (not to be confused with the middle control) a generous amount, which adds fullness and brings it in line with the clean and interactive modes. I've concluded that an investment of $150 in speakers will be a good one, so that's next. As mine is now, I rate the BC30 a 7.5 on a scale of 1-10. A good set of speakers could push it up to an 8 or higher. I'll report back when the speakers are installed and tested.

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hi all! thanks for the wonderful tips here - i like the sound of the mods!

 

after my debacle with the ornery CH1 volume pot, i decided, what the heck, ill do them mods.

1) C5, C6 one leg each cut. decent treble tamed for CH2.

2) Parallel a 5.6K/1W resistor with R15. WOW! CH2 is more alive, brighter drive, more defined distortion. some slight increase in perceived loudness too.

3) C3 on leg cut. CH1 sounds less treble-y. easier to manage!

 

thats all ive done on the circuit bends. And all tested with stock 6L6GC and 5AR4, but has old stock mullards for V1 and V2, old stock RCA for V4, and old stock telefunkens on V3 and V5. I decided to use the stock power and rectifier tubes since theyre cheap, and i wont lose any sleep if i end up breaking them. those NOS 5AR4's are $$$$$!

 

but i must say, you guys are geniuses to have figured out what to tamper with to give it a better tone - THANKS!

 

as suggested on one of the posts here, i m looking to put 3 small switches to have the option of playing this stock, or with those mods. Im looking to put a switch for C3, one for the C5/C6 cuts, and one for the R15 paralleled resistor. man, that resistor is the best sounding, most noticeable among the mods mentioned.

 

though not technically a mod, up next is TUBE ROLLING!

im using old stock sylvania and raytheon 6L6GC's as my default power tubes. My default tube rectifier is a NOS Bendix 5AR4.

- solid-state rectifier (old stock US 6L6GC's are strong enough to withstand the higher voltage, so no worries about it. id be more afraid for the stock EH tubes)

- old stock tung-sol 5U4GB tube rectifier. i already know how this sounds stock, but we'lll see what the mods will do...

- a couple of NOS NIB 5V4's tube rectifiers.

-- ive ordered several NOS NIB russian 6P3S and 6P3S-E tubes just for kicks. plus theyre dirt cheap

- Yellowjackets! Curious to know how EL84's will sound on this amp.

 

im leaving the lady luck speakers in. i kinda like its tone, BUT....

seeing as theres several different speaker outs on the back, I may hook this up to a couple of 2x10 cabs with '66 or '67 C10N jensens.

Then see how it sounds on a couple of 1951 25W 12" alnico jensens in its 15W mode.

 

cant think of any other mods id want to do, but again, thanks for the useful knowledge, guys!

 

- Marc

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Hello Friends,

I just did the C5 clip, and its a GREAT DIFFERENCE!! I think I will stay there for now. Also swapped the power tubes to Tungsol 5881's and am waiting for tungsol 12AX7's I ordered,and Sovtek rectifier.I also am swapping the power cord to 14AWG. Thanks to Papamidnite for the awesome thread and my bro el_pabst for walking me through the disassembly and mod. All of the contributors get a huge ovation for their input...[biggrin] Thanks, now I know what to do down the road if I want to mod even futher..........Very best regards,

Michael

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Wow, I'm glad to see this thread has grown way beyond My original ideas.It looks like you Guys nailed it .I was thinking of grabing another one But at the moment I'm having too much fun with an Ampeg .Oh buy the way these are showing up at studios all over the place . Keep on playin that rock and roll ......

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Wow, this is one of the most successful mod threads i've ever seen. :-)

 

My questions don't quite relate to how most people have been using this amp, so i'll throw it in here -

 

1. I ONLY use the clean side (@ Class A 15w), as i use a tube distortion pedal for all my gain. Because of this, the only Mod i'll probably do a is C3 cap clip - HOWEVER, i would like to know which circuit mod was used to maximize use out of the interactive EQ. At this point it's pretty much unusable do to the dramatic volume drop once it's pressed...weirdness....the mid frequency is in a weird place on this amp...

 

2. What would be the best way to get more headroom out of the clean side of this amp? I'm thinking about using some higher wattage 6L6gc's, like 7581a's (which can draw up to 35w)....would any adjustments be needed for tubes rated beyond 5881's? I know now it is cathode biased....

 

Have also thought about going even higher, maybe to 6550's - would this amp's PT be able to handle this? Has anyone tried other tubes besides 5881's and EL34's?

 

- I've already done a little tube changing, which i'm pretty satisfied with; the theory behind it was based on putting a 5751 in the final output stage to help maximize use out of the power tubes -

 

V1 - Vintage RCA short Plate

V2- GE short plate

V3 - Stock svetlana

V4 - Strong rated Shuguang 9th gen (110/110 gain rating new)

V5 - JAN/GE 5751

 

This seems to let me pedal slam the amp a little bit harder, but i just can't get any more clean power.

 

Enjoyed reading all of your guys input and modifications, let me know what you think. Thanks!

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Um, the amp is Class AB1 in both modes (doh!) it is switched either triode of tetrode, even EL34 are run as tetrodes. You /can/ design a push-pull to run as as pure Class A, but no guitar amp does that. If a valve switches off at /any/ point of its cycle then it is AB. If the valve does not switch off then you don't get high power, guitar amps are all about high power output.

 

Maybe because the BC30 is 230VAC and UK mains measures 245V, I'm getting about 430V HT and -37V on cathode. Svetlana 6L6GC data indicates this could be in the order of 50W. Given that my Greenbacks are only 98dB and this amp is incredibly loud in a live situation (we have to keep it turned down or we drown the band), I think we're getting well over 30 watts. Triode mode is rather loud too but tonally different.

 

The eq is pretty weird and very interactive. I have virtually undone all the mods I did to it. My normal setting is treble 9 o'clock, middle and bass around half-past 10 o'clock. The pots are large-value log where most T-bridge use lower value linear. Turning the middle to max removes it from the circuit to all practical intents.

 

The gain channel can be run fairly clean by dropping the gain knob, then you get the use of that middle/presence control, I use it almost maxed, gain 11 o'clock, volume to suit. N.B. I have a Master Volume, and I really really recommend this mod.

 

Svetlana =C= 6L6GC are huge sound-wise. For clean, I don't think you need any more headroom than these but they do have a big bottom.

JJ 6V6GTS=6V6S, a high-voltage almost 5881 look-alike type. No noticeable loss of volume compare to stock Sovtek 6L6WXT+. Maybe not as much headroom but punchy and sparkly clean, tight bottom.

 

To me, 5751 spec don't look like a good substitute for 12AX7/12AT7 in the PI. I have used a Mullard ECC81 (12AT7), and quickly put the ECC83 back in :-(

To get extra clean try an ECC83-STR or E83CC (i.e. a super-clean low-microphonic type) in V1, a higher than normal gain selected type here will also push the drive channel a bit more.

 

The amp is cathode/self-biased. Without re-biasing you can safely use any octal 5881=6L6WGB, 6L6GC, EL34 (& KT77/88?, 6550) etc provided that they can take the HT volts which is over 400VDC. I have used JJ 6V6S in there. But, but you will not get much increased power just by swapping the bottles, you would have to re-bias specifically. Of interest, the mains transformer has a tap for grid bias, as per the SoCal50. I opine the Beastly30 needs no additional volume, thanks.

 

Regarding the bright/boost switch. I used a sub-miniature 2p2w toggle.

Originally I used it as a bright on both channels but changed it over to a bright switch on channel 1 and a boost switch on channel 2.

Bright is up, it puts the cap back down (for my mods see above). I am still undecided whether up or down for fat boost on channel 2 is best, but used with the channel switch you get four settings, roughly jazz, country, rock and blues, and then end up using fat boost with the gain down for jazz.

 

I will take the opportunity to reiterate that changing the Chinese rectifier for a Sovtek or practically anything-else is a good thing.

 

Imo the stock speakers are harsh and this is a harsh amp, so good old woody Greenbacks suit it, because they're not.

 

Master volume, master volume, master volume. Yup, this really, really tames the amp. It allows you to crank the pre-amps and take that shriek off.

 

BTW I am posting these notes for folks like me what like fiddling with their amps because out-of-the-box the BC30 is a bit of a PITA.

 

 

Can anybody explain that output transformer ? Is it really 22kohm - 0 - 22kohm ? Is that 44,000ohm for plate-to-plate, OMG ? Svetlana reckon 6L6GC Ra-a ought to be 5600ohm. Somebody please tell me they got the decimal point wrong on the transformer label ! What on earth is going on there ?

PLEASE ?

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As I'm sure you're all aware, Channel 2 has always had less bass than Channel 1. This is largely because the extra 'Midrange' control can never be fully turned off, it always filters some bass out. You can fix this by removing C14 thus disabling the 'Midrange' control. You could alternatively replace VR3 with a pot that breaks the circuit when fully turned.

 

Note there is still a little more chime on channel two, probably because of the extra valve stages with cathode bypass etc...

 

;-)

Andy

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Hey all. I'm new here but not new to my BC30. I've had it for about a year or so and I'm really interested in trying the C3, C5, & C6 mod but I have one question. Does it make a difference which leg you cut on those? I know it may be a dumb question but, you never know.

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When removing bright caps you don't just cut a leg, you carefully desolder and remove the capacitor completely because otherwise vibration in the amp may cause it to short back into the circuit and crackle badly. Also if you dont like the change you can simply put it back (assuming you havent cooked it with slow work!). Any cheap solder sucker (desolder pump) will really help. If you want to try it, then remove them completely.

 

Also you must realise the the big filter caps can and will hold enough charge to kill you, so before starting work simply use a wire with insulated crockodile clips to short the anode/plate resistor of valve1 (the one nearest the instrument input) to the chasis. Within a couple of minutes the resistors in the amp will have drained the filter caps.

 

BTW I've got a fair bit experience with modding this amp and the bright caps are good as they are. If yours has ice-pick syndrome it's likely to be because of the valves you're using, not the bright caps. Cutting the caps really only ruins the overdrive, sucks the life out of it.

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BTW I've got a fair bit experience with modding this amp and the bright caps are good as they are. If yours has ice-pick syndrome it's likely to be because of the valves you're using' date=' not the bright caps. Cutting the caps really only ruins the overdrive, sucks the life out of it.[/quote']

 

Thanks for tip. I had actually planned on swapping out the valves before I did any modding.

 

On another note, any thoughts on adding an effects loop?

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I've recently started foolin' around with caps and resistors in the reverb section. I clipped r29 and I am liking the tone of the reverb a lot more. Any of you brave enough to do the same and compare?

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Can anybody please explain that output transformer ?

 

Is it really 22kohm - 0 - 22kohm ?

 

Is that 44,000ohm for plate-to-plate, OMG ?

 

Svetlana reckon 6L6GC Ra-a ought to be 5600ohm. Somebody please tell me they got the decimal point wrong on the transformer label ! What on earth is going on there ? [traditionally RL = 2Ra (single valve), but halve that for push-pull, hence RL > Ra-a approx]

 

 

PLEASE ? anyone...?

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Tone control: more-or-less stock now, but the slope resistor R22 is now about 33k and I've added a 22nF cap from its junction with C17/C18 to the bottom of the bass pot, and R23 which is now about 10k. This makes it more like a conventional T-bridge, and more usable (for me) the knobs do something.

 

I've noticed similarities to the WEM Dominator /"18 watt" Marshal and the 20W Marshall "2204"...

 

Ch2 midrange tone cap C14 is 22nF, the 18W Marshall and WEM Dominator single tone knob types use a 10nF here, just thinking... Anyway tried a few different values incl 100nF in there now, makes little odds.

 

Harshness and lack of bass: R5 and R6. Hmm, take a look at a 20W Marshall, these are voltage dividers with their VR but their fundamental frequency is governed by the caps C3 and C5 =470pF - no wonder this amp is so harsh... The Marshall 2204 uses a 470k here, that halves the frequency [F=1/(2piCR) ]. Ok, so I shunt the 1Meg with 820k, makes about 450k, and lo! lovely bass and fullness of voice emerges. I recommend this one!

 

Enjoy!

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Oops!

 

I've maybe advocated modifying the cathode resistor on V2A above but the switch may actually be on V1B. I have modified both at some time and I'm afraid that my diagram has so much scribble on it now that it is hard to tell. When I look under the hood next I'll sort that out and re-post.

 

I have now A/B'd the BC30 clean alongside my lovely sounding WEM Dominator 25 (with original Mullards) and I can say that we are defiantly in the same ballpark now tone-wise, about 97% I put this down to the "20W Marshall 2022" mod to R5 and R6 changing the frequency response across their 470pF into the second stage. My strange mods to the T-bridge also helped a little, the rest is down to the speakers.

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Hi guys,

 

I did the C6 and C5 mod, so much more bass now, the tone knobs actually make a difference now. I dunno why they didnt build them like this is the first place. I just unsoldered one of the legs on each cap incase I wanted it back again.

 

See how things get on gig wise before I think about removing C10.

 

Cheers

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Hi there..

 

Just wondering.. with the cap mod, by 'cutting', what do you mean? Snip one leg with wire cutters? Both legs? De-solder completely? I've been reluctant to do this so far as I'm afraid it will make my SG and Explorer sound too dark. But I'm getting too curious now, especially after reading other people's feedback after doing it. I'm gonna try it this afternoon, as soon as I hear about how people are doing it! Thanks in advance.

 

Danny

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