Blues Custom Mods

[myersbw]

This is my first post to the forum. And, this has been a great thread for me. I've just acquired a BC-30 that has some hum issues, but it was a freebie. :) I'm also a 30+ year tech so I'm not opposed to mods. I've loved all the input this thread has offered for tweaking tone out of this lil monster. That said, I've taken a different approach. I did see one individual (different thread/forum) that popped in a 5YG3 rectifier & some 6V6GT's and has worked.

 

Peeking at what I have in the spare parts bin, I ended up popping in a 5U4GB and a pair of 6V6GT's (old RCA stock). Given the extra voltage drop the 5U4 gives over the 5AR4, it brought the B+ close to the design max for a 6V6GT. No immediate problems...tone was AMAZING. but, I still need to reassess the differences and maybe tweak the power supply circuit to run a bit more efficiently & avoid any extreme plate voltages.

 

Overall, I like it. May go with the ElectroHarmonix 6V6EH (475V tube) or JJ's 6V6S to ensure I don't see any red plating. Anyone else ventured to 6V6 land in here?

 

The amp does suffer from hum I need to troubleshoot. But, I know at least one source of a bright buzz is a bad standby switch (confirmed). Overall, it's a nice lil amp & nice for tweaking...ouch, but SO heavy!!! :)

 

Cheers!

Brad

[Andyroid]

I tried a 5Y3GT and a pair of 6V6GTs a few years ago and had hum and a little red plating. I fixed the red plating by increasing the cathode resistor, but lost the amazing tone in doing so. If I didn't need to look after my 6V6s (I use them in my main amp) then I would try again and add bigger screen grid stoppers instead. Just an idea I thought would be worth mentioning!

 

I think the hum might be coming from the 5Y3, you could try the 5Y3 with your 5881s to check that.

 

Another thing, my FAVORITE mod is a pair of recessed side handles. Makes the amp so much easier to lift, especially when you make it a two man job!

 

Have fun!

Andy

[myersbw]

I tried a 5Y3GT and a pair of 6V6GTs a few years ago and had hum and a little red plating. I fixed the red plating by increasing the cathode resistor, but lost the amazing tone in doing so. If I didn't need to look after my 6V6s (I use them in my main amp) then I would try again and add bigger screen grid stoppers instead. Just an idea I thought would be worth mentioning!

 

I think the hum might be coming from the 5Y3, you could try the 5Y3 with your 5881s to check that.

 

Another thing, my FAVORITE mod is a pair of recessed side handles. Makes the amp so much easier to lift, especially when you make it a two man job!

 

Have fun!

Andy

 

NICE on the handles! If I keep this amp (and likely will), I may do this. If I do, I can cut out and insert a screen in the top for convection and toss out the fan. Really, my fave tubes are EL84's so, ultimately, I'm not sure where I'll take this. A lil BadCat clone might be in order, too. :)

 

What brand 6V6GT's did you use?

 

Brad

[myersbw]

BTW, did I miss them? Or, does anyone have some inside chassis shots they wouldn't mind sharing?

[Andyroid]

I used modern Tung Sol 6V6GT. So they were operating way out of their league. I've heard the modern JJ/Tesla 6V6 would probably handle it but would sound like a cross between a 6V6 and a 6L6. As for the EL84, I know someone round here has had great luck with the adapters you can get, Tone Bones or something like that.

 

Don't have any inside pics, sorry.

 

Just noticed you mentioned the standby switch, you can safely hard wire it in the on/play position. There is no need for a standby switch on this amp, in fact it's affectionately called the 'self destruct switch' by many, as it can and will destroy your rectifier valve and blow fuses.

[jefrs]

I have used JJ-6V6GTS aka JJ-6V6S in there with no problem at all but have not / would not crank it right up. No red plate there. Note our mains is almost 250VAC so the 240Veuro PT will be running HT B+ a wee bit higher than spec hence 34.3V seen. Usual and current op valves are =C=6L6GC it's not just loud, it's bloody loud!

 

I have taken chassis pics but cannot remember if I ever posted them on here. It will be the same as yours with a bit of tidy-up plus some damage from dropping. Let me know if you really, really need them?

[myersbw]

Ok - back into the garage tonight to troubleshoot. I haven't received my replacement standby switch, but it will not get reconnected. The old leads pulled completely out when removing them. I patched the wires together & fired it up with some vintage RCA 6V6GT's and a 5U4GB rectifier.

 

The B+ at the plates dropped to 362V with 21V dropped across the cathode 250 ohm resistor. Total screen/plate current per tube is 41mA at idle. So I'm dropping about 341V anode/cathode. So far...so good. These tubes are holding up. All my previous hum is completely gone. If you have hum...suspect the contacts pitting in the standby switch.

 

Epi is sending me a new one as a courtesy, but I'll leave it non-functional. I will, in all likelihood, mod this for EL34 use this Fall. Love the 6V6GT tone so much I'll likely leave them in and use it as an acoustic amp. I'm never likely to push it. Might pop 6L6's in, but I'll grab a new rectifier before I try that. Don't really want to leave the B+ low and push the current up.

 

Anyways...thanks for the hints, mods, helps, advice. I was going to gut it...now I'll just mod it a lil. :)

 

BradM

[jefrs]

Do please note that my main "tone opening up" mod was to put 820k shunts around both R5 and R6 (both 1Meg), reducing it to 450k.

The logical reason for using a 1W resistor here is that they are longer and can thus "stand off" a higher voltage, not that they pass much current, even if they had the entire HT across them, they would only see 0.18W P=E^2/R.

 

My C4 "bright cap" is now on a bright switch. However the CH2 "bright cap" C6 is disconnected. Do check component values here, I found some on the PCB that did not match the circuit diagram.

 

Also, for interest, my B+ appears to be 420VDC with the cathodes at 34.3V wrt ground.

 

I also separated the HT from the heater wires, and twisted them. And another note that I separated the speaker socket ground so it does not return through the main PCB. Why would you pass that current through the main PCB? It's supposed to go to the O/T. These things do make the BC30 hum less.

 

I did post up a circuit diagram here, with my mods on it but it was only as large are permitted and so may not be entirely legible.

I can read it, it pops up very large and you can determine the original circuit.

That's at post#145 - http://forum.gibson.com/index.php?/topic/43505-blues-custom-mods/page__view__findpost__p__758439

 

I have A/B'd mine with an EL34 mod BC30 and imo not much to choose between them, very adjacent. Actually they only modified for EL34, not the R5/R6 shunt so it still sounded congested. In theory, the EL34 is supposed to make it a trifle louder. In practice the little mods I made to mine made it too loud, too easy to drown out the rest of the band, hence the MV. =C=6L6GC

[myersbw]

Here's a peek at my bad standby switch...note the pitting from arcing, lol, also note the missing posts as they pulled right out. (really poor switch!) :)

 

 

 

BradM

[myersbw]

So, the new standby-not-using switch came in today. Here's a summary of my issue with the amp, solutions & mods done:

 

1 - Hum & crackle issue when I got it, yet worked. Swapped tubes, same fireworks. Culprit? Toasted (severely pitted) standby switch. Epiphone sent me one as a courtesy (thanks!!!), but the same model as the failed one. I installed it and will just remember for now to continually leave it on. Amp is restored to pristine quiet.

 

2 - Pulled the original 5AR4 and installed a 5U4GB and installed a vintage pair of RCA Black 6V6GT's. Measurements are as follows,

 

6V6GT plates - 362 V

Cathode resistor drop - 20.7 V, 20.7/250 ohm = 82mA or 41mA tube current at idle including the screen current. Thus, the tubes are dropping 341V

- so far, no redplating noticed with the lights out and playing for 5 minutes. More careful watch to follow.

 

3 - At this point, all operations are fine. Time to do mods. Using a Strat with a Duncan Hotrails at the bridge for "shrill" testing.

 

- C5 clipped - ice pick is gone. Clean channel has good tone swing, but the OD channel is too sterile.

- R15 - wow! - factory mistake! No wonder my tone sucked. This 2006 model has a 10K cathode resistor for R15 (NOT 2.2K or 1.5K). The other preamp cathode resistors were already 1.5K from the factory. I just had a pair of 4.7K's so those paralleled with the 10K parked it at 1.8K. Unbelievable difference. That gain boost brought some fire into the tone. Gain was nicely saturated & balanced.

 

The 6V6GT's are one's I'll likely keep in this. May pop in 6V6S' next for comparison. Chimey clean and smooth bite in the OD channel. Nice chord definition.

 

Wrapped it back into the case...stock Lucky Lady speakers. Not a bad tone. I'll keep it this way. I did make a pair of chassis supports to make working on this fairly easy. I used what was on hand...nothing fancy (see the image attached).

 

Thanks to all of you that have given me so many areas to pursue modding! My total mod time spend came down to about 2 hours. (Eh, with about 8 hours of various days of "what-if's" pondering all the options.

 

I'm guessing I could make this work well with EL34's if I removed the reverb tubes & swapped in a GZ34...might have a nice current balance then...? Will explore that later...

 

Cheers All!

BradM

[jefrs]

My standby switch actually exploded out of its back. It is an AC mains switch being asked to perform at >400VDC, no wonder it arcs.

Completely the wrong tool for the job. It is also an odd size. I used some big "repair washers" to fit my MV there.

 

You do not need a standby switch, you /can/ bypass it, spade connectors are easy to do this to. Just use the on/off switch and silence the amp with the input socket (it shorts = silence) like you are supposed to do. The standby switch is a fashion accessory.

 

The Chinese rectifier in mine also made pop-corn noises, that was arcing over too, rubbish bottle. Sovtek 5AR4 very good and can do EL34

The stock EH ECC83/12AX7 were ok except I strangely prefer the cheaper Sovtek flavour (from same factory in Saratov?), although the EH were best for, and are still on, the reverb. I have since completely re-valved the other ECC83 with a variety of bottles, notably Harma's "Retro" Mullard clones (also suspected from Xpo-Pul/Reflektor)

 

I found some "wrong value" components in mine too, but not a cathode resistor. Bad luck. Possibly worth checking a few others.

 

You may toast any 6V6 (max plate 315V) except JJ-6V6GTS = JJ-6V6S. The JJ appears to be a 5881 with a low current heater that sounds like a 6V6, it can take something like 800V on the anode plate, I'm not sure how much current (power) it can handle though but I've not managed to blow my pair up. Putting 6V6 in does not make the amp much quieter, cranking it up will likely take 6V6 over 26 watts, which won't do them any good at all. The 6V6 has a low power or "half current" heater because it was intended for use in vehicles with pathetic 6V batteries.

Note - I am getting 420VDC because I've got very close to 250VAC on the PT set to its max 240VDC. That means I get a higher bias point too.

The fixed bias point for 6L6 is -37V at 450VDC, so -34V is a good compromise for 6V6/6L6 at this B+. EL34 would want -36V. The cathode resistor self-adjusts somewhat, it's about getting that bias point almost-right that counts. We are running much higher B+ than most spec sheets, hence greater bias volts. See plate characteristics graphs ;)

 

BTW - using the full winding of the OT, 16ohm into 16ohm speakers, sounds crisper and clearer than the 8ohm tap (into stock pair at 8ohm)

 

FWI - "pin 1" should already be connected to cathode so it should take any octal incl EL34 although a change of cathode resistor might be advisable. Although imo the =C=6L6GC sound more open, clearer and "bigger" than the EL34 mod BC30 I know.

[myersbw]

I've plugged it in...will never switch it though. I may bypass it if i find I want to add an MV there. So, yes, I just toggle the Power switch only.

 

Actually the 1960's gray glass spec for the 6V6GT's by RCA were a design max of 350V and 14W. Mine drops 342 across the tube. The tone is fabulous on them. But, now that I've fine tuned the OD channel, I'll likely drop the 6L6's back in that I have. (Yes, my 5AR4 was arcing when I got the amp (used), too).

 

I've been emailing Pyotr about the model I have (black circuit board vs. green ). The 5U4GB drops that B+ down by 45-50 V from what the 5AR4 provided. What I may do is...go solidstate so I can remove the rectifier tube and make sure I don't exceed the extra heater current needed should I go with EL34's. Yet, running the other direction might mean tweaking a bit more in the preamp. In all likelihood, I may not fuss anymore with it at all. But, may pursue some decent 6L6 or 5881's to pop back in...that 6V6S is a bit interesting though.

 

BradM

[jefrs]

You can get a plug in solid state rectifier that directly replaces the 5AR4/etc bottle.

example http://www.watfordvalves.com/product_detail.asp?id=3071

 

Expect to get more volts with solid state diode rectifier.

[PapaMidnite]

Damn you guys are deep into this one [Lol ] It's funny to see this thread still going . I like the 34 ideas I have an amp that I bounce back and forth with 34s . But awhile back I fell in love with 6550s It pushes the amp hard had to add an aux. fan to keep it from blowing up haaa.But anyways nice work on this one

[myersbw]

Damn you guys are deep into this one [Lol ] It's funny to see this thread still going . I like the 34 ideas I have an amp that I bounce back and forth with 34s . But awhile back I fell in love with 6550s It pushes the amp hard had to add an aux. fan to keep it from blowing up haaa.But anyways nice work on this one

 

...well...I got mine a month ago so all this is new to me. :) Pyotr has been great with some email correspondence on the amp. He affirmed that the transformers were the same as the SoCal....it gives a 4K load. He wouldn't change any of the resistors unless tweaking...should work straight up with EL34's without changing them.

 

Now, I did read up a bit about typical Miller capacitance and blocking resistors. One school of thought is to take them up to 15k-45k to get that 3db reduction cutoff freq down into the 35KHz-45KHz range. And further helps eliminate blocking issues. I may try that.

 

I also popped my 6L6GC's back in it. Eh...clean just makes for more overhead vs. the 6V6's...I can't really hear a diff. But, I may like the OD a bit more. So much that I'm now convinced I'll push this to EL34 territory and sort of BlackCat it...moreso like the HotCat 30 since it's close.

 

BradM

 

 

[jefrs]

Damn you guys are deep into this one [Lol ] It's funny to see this thread still going . I like the 34 ideas I have an amp that I bounce back and forth with 34s . But awhile back I fell in love with 6550s It pushes the amp hard had to add an aux. fan to keep it from blowing up haaa.But anyways nice work on this one

 

Yes, it has been running a long time ;) - but it ain't dead yet!

 

One of the first things I had to do when I got mine, many moons ago now, was connect the fan because there was a wire off at the PCB. Then I observed it made the amp hum and had to re-route the fan wires as twisted-pair. The fan supply should not be taken from the middle of the PCB near a valve base. Part of the heat 'problem' is lack of sufficient vents and running the bottles 'upside-down'.

 

The fan is a "computer" fan, my PC has additional fans (controlled by temperature) due to old house dusty environment and cats. The main CPU now has two mounted in tandem.

 

How did you attach your aux fan?

 

 

The 6550 is interesting. They can take an amp to 100W territory. Did you do any mods to increase power or just 'stick them in'?

I find my BC30 already too fearsomely loud for a lot of use. I have no real idea what its output is with the =C=6L6GC but I suspect well over 30 watts. Hence no wish to increase power further.

Ime not all 6L6 types sound alike, there is a lot of variation. So worth playing with different makes to find ones one likes. I reckon I've got my BC30 well into Bluesbreaker territory now.

[myersbw]

Well, I ordered a pair of EL34's & diodes today. Also, a 5AR4 to test, but feel the diodes might keep the drain on the heater supply at an acceptable level. Will report what I find!

[jefrs]

Well, I ordered a pair of EL34's & diodes today. Also, a 5AR4 to test, but feel the diodes might keep the drain on the heater supply at an acceptable level. Will report what I find!

 

By drain on the heater, do you mean the valve rectifier heater? - that is supposed to be supplying power, and quite a lot of it, to the amp. It is a feature, not a fault.

 

If you mean the other valves' heaters then that is taken off a separate winding on the P/T, it is unaffected by the rectifier whether valve or solid state.

 

Now the pre-amp bottles run their heaters in series from a supply of +13.5VDC on mine (due to my high mains volts again) but the output valves have 6.3VAC supplied heaters.

[myersbw]

By drain on the heater, do you mean the valve rectifier heater? - that is supposed to be supplying power, and quite a lot of it, to the amp. It is a feature, not a fault.

 

If you mean the other valves' heaters then that is taken off a separate winding on the P/T, it is unaffected by the rectifier whether valve or solid state.

 

Now the pre-amp bottles run their heaters in series from a supply of +13.5VDC on mine (due to my high mains volts again) but the output valves have 6.3VAC supplied heaters.

 

Basically, I'm referring to the rectifier tube's heater consumption for it's own operation. I.E. ... has it's own heater that is consuming power from the PT. Much more than diodes would consume. If I pop in diodes, I'm not wasting power from the PT for the tube rectifier's heater. All that contributes to load on the PT. As the B+ goes up due to the diodes (no tube rectifier loss), the I^2*R losses will reduce overall, too. I'm hoping I'll like where the B+ sits and the tone of the EL34's. If not, I'll pop a 5AR4 back in (replacing the 5U4GB I have in there now). The 5U4GB draws a little more heater current than the 5AR4. The silicon diodes add no extra heater load. That is the load I'm recovering back for the PT.

[jefrs]

Basically, I'm referring to the rectifier tube's heater consumption for it's own operation. I.E. ... has it's own heater that is consuming power from the PT. Much more than diodes would consume. If I pop in diodes, I'm not wasting power from the PT for the tube rectifier's heater. All that contributes to load on the PT. As the B+ goes up due to the diodes (no tube rectifier loss), the I^2*R losses will reduce overall, too. I'm hoping I'll like where the B+ sits and the tone of the EL34's. If not, I'll pop a 5AR4 back in (replacing the 5U4GB I have in there now). The 5U4GB draws a little more heater current than the 5AR4. The silicon diodes add no extra heater load. That is the load I'm recovering back for the PT.

 

Yes it has its own heater but it also forms part of the power supply, it is not wasted. That current drawn from the heater goes into the HT. It is supposed to do that. That heater is also acting as a cathode.

It is not just heating the cathode but supplying power too. That is why it has its own high current supply from the PT.

[myersbw]

Yes it has its own heater but it also forms part of the power supply, it is not wasted. That current drawn from the heater goes into the HT. It is supposed to do that. That heater is also acting as a cathode.

It is not just heating the cathode but supplying power too. That is why it has its own high current supply from the PT.

 

Yes, that was a difficult thing for me to find. I actually found a really good article (it was late...lost it ...will post a link later) where the discussion was that...yes, you can pull out a rectifier tube and reduce the load by 10-15 watts of consumption based on a given tube. But, the other losses where great enough with the elevated B+ that you ended up with more actual heat dissipation in the power supply due to the conversion to SS diodes.

 

I should have a 5AR4 and a pair of EL34's awaiting me when I get home this eve. :) We shall see how they sing!

 

BradM

[jefrs]

Yes, that was a difficult thing for me to find. I actually found a really good article (it was late...lost it ...will post a link later) where the discussion was that...yes, you can pull out a rectifier tube and reduce the load by 10-15 watts of consumption based on a given tube. But, the other losses where great enough with the elevated B+ that you ended up with more actual heat dissipation in the power supply due to the conversion to SS diodes.

 

I should have a 5AR4 and a pair of EL34's awaiting me when I get home this eve. :) We shall see how they sing!

 

BradM

 

It may take a moment to get the head around it but that 5V(?) high-current heater is also firing hot electrons onto the rectifier anode. They are supposed to do that and why a valve rectifier cannot be run off an ordinary heater circuit. SS diodes are more efficient.

Increasing B+ will alter the bias requirement. Cathode bias can cope with a lot of variation but the one in there is intended for 5881, Rk (and load/OT) on EL34 is different. Keep an eye on the EL34, see how they glow.

Can you safely measure B+ and bias volts?

 

I don't think valve amps are designed as energy efficient appliances.

[myersbw]

It may take a moment to get the head around it but that 5V(?) high-current heater is also firing hot electrons onto the rectifier anode. They are supposed to do that and why a valve rectifier cannot be run off an ordinary heater circuit. SS diodes are more efficient.

Increasing B+ will alter the bias requirement. Cathode bias can cope with a lot of variation but the one in there is intended for 5881, Rk (and load/OT) on EL34 is different. Keep an eye on the EL34, see how they glow.

Can you safely measure B+ and bias volts?

 

I don't think valve amps are designed as energy efficient appliances.

 

Measurements...not a problem. I've got a pair of homebrew chassis stands that let's me prop this thing vertically to make mods, fully test and all plugs in well. (A pair of 'T'-legs that utilizes the chassis mounting holes...suspending it control-side down. (scroll up the posts and you'll see my chassis on the stands)

 

The parts have arrived. (It's also awesome to be within 30 minutes driving of Parts Express if need me.) :) I've actually ordered what i need to near replicate the BadCat Hotcat 30 output section. I'm also going to park a modified L-Pad at the output, too. In the late 70's, I had an original '72 Orange 120W head that would just sing when you saturated everything and hung an Altair Power Attenuator off of it to pad it. (Yes, I'm trying to keep my tinnitus levels to NOT increase more than what they are.)

 

So, planned weekend mods, changeout the Cath resistor, blockers, etc., install a 5AR4. The tubes I grabbed were Rubys. They were cost effective to experiment with. If I like the changes, I'll likely go grab Mullards & drop in. I don't play it enough to not get a long time out of them.

 

Cheers! ...will report back after the changes!

BradM

[fuse]

I forgot all about these amps.

I remember they were pretty popular when they first came out.

Have always wanted to get one "on the cheap"...but never saw one pop up on my local CL.

Some day perhaps.....

best

[snowblind]

Hi,

 

New to the forums. Liking this thread lots.

 

So... my (sort of - badge on the front says "Grainger") BC30 has a duff standby switch and for various I want to replace it rather than dumping it. So I take the switch out and it has a bezel 25mm x 25mm. It appears that no one on the planet makes switches in these dimensions. Global standard seems to be 25x31. Anyone know where I can get a replacement switch to fit or do I just start carving the front panel to take a standard item?

 

I did email Gibson support who (bless 'em) pointed me in the direction of "British Audio Service" which was nice as I'm based in the UK. Slight problem in that B.A.S are in Nashville, TN.

 

Cheers