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BC 30 valve positions


RickJ

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Could anyone clear up the exact functions of the valve positions? I know that V3 & V5 work with the reverb. But exactly how and in what capacity? V1 & V2 a and b are preamping the 2 channels and possibly performing a gain make up after the tone stack. But exactly which/what? V4 a & b is the phase inverter, and along with V6 & V7 comprise the power stage. But what exactly happens here when the toggle from 15 to 30 watts is used? And V8 is the rectifier.

 

I also have a perfectly balanced NOS Tung Sol 12AX7 reserved for the PI position but which end of the trio of preamp tubes is it on? Near the output transformer (on right as looking back to front with back off) or on the power cord end (on left same orientation looking forward from rear of amp)? I'm assuming the two 12AX7's toward the front of the chassis are V3 and V5 and the rear trio next to the power tubes are V1, V2, and V4. But I need to know which end V4 is on.

 

Thanks,

Rick

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Not true! The schematic lists the component numbers, which are marked on the board by each component. As long as you've got the schematic as a guide and a good meter, you've got the keys to finding your way around the circuit layout on the pcb.

 

BTW, the PI is V4 (V4a & V4b), so look for the parts directly associated with that tube. No big deal.

 

Gil...

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Oh yeah. Tube functions.

 

V1a is the input gain stage your guitar gets to tickle. The signal is then redirected by a relay channel switch, so one "channel" goes through V1b to V2a (which drives a midrange tone control). The other "channel" just bypasses the V1b+V2a gain stages and mid tone. Both "channels" then hit the other side of the relay switch again just before V2b, which drives a tone stack. After the tone stack, the signal is then split by a switch for feeding a portion of it to V3a and V3b (in parallel) which drives the signal into the reverb OT's primary winding, which is magnetically coupled to the secondary winding loaded by the spring tank. The relatively weak signal from the spring tank is then boosted by the recovery gain stage V5a and V5b (also in parallel), and mixed back into the main signal right before the PI V4a and V4b, which is setup in Long Tail Pair phase inverter configuration, which feeds the power tubes V6 and V7 which feeds the OT which tickles the speaker!

 

 

And there ya go. One BC30 playbook, tube by tube.

 

Gil...

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Excellent, there's half of what I'm looking for. But the schematic IS useless for physical layout unless you pull the chassis out to look at the board, which I really don't want to do just for a tube swap. It seems the V designations aren't printed on the outside of the chassis. Now all I need to find out is which end of the trio of 12AX7's holds the PI in V4. Once I have this info, I'll be able to swap in my balanced dual-triode and won't need to pull the chassis. I'm not an electrical engineer I'm a mechanical engineer...oh, and a musician.

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This is exactly what i need.

I want to replace tubes on my BC30...it's just time.

I need to know from one end to the other which position is which so I know I am replacing in the proper order.

I will send a PM for the schematic.

I have no issues taking out the chassis (I did it once to do a clip of resistors with great success).

Thanks!

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Many thanks to Layboomo, i now have the schemo!

So here's my problem, like others, I'm not an engineer.

What I'm trying to determine is what tube in each position does what.

So I need to know using a reference point on the amp or looking at the line of tubes, which is which.

IE...left to right or closest to the power tubes to furthest from the power tubes. Something like that.

I see references to V1 and Va and b...I don't know what those mean other than i assume V means "valve".

I know this stuff comes easily for some, just not me!...haha

Ideally what I'd like to do is open the back, pop off the respective tube and pop in an upgrade or replacement for each.

Should be easy enough.

Can a BC30 owner help me there?

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Perhaps an inverse of a previously stated general rule of thumb will help? Try this on fer size...

 

The first preamp tube, V1, is virtually always the one found closest to the input jack (or set of jacks) that is farthest to one end of the control panel.

 

There are special cases (Marshall 1974x, for example) where V2, V3, etc. may not follow a logical order of progression in terms of their physical location on the chassis, but this is probably not one of them. However, to be absolutely sure, one could always simply pull the chassis and follow the circuit on the schematic while matching each valve stage's related components to the component numbers marked on the pcb. :(

 

Gil...

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I had to dig deep in the forum for this one!

 

V1 = CH1 preamp

V2 = CH2 preamp (fed from CH1 preamp)

V3 = Reverb in

V4 = Phase Inverter (PI)

V5 = Reverb out

 

 

Looking at the amp from the rear as it is mounted in the cabinet the #5 position is the one on the left next to the white reverb connector. You can count backwards from there as the tubes are lined up numerically. Oops! Forgot to add that the above does NOT include the rectifier or the power tubes. You will be looking at and counting only the smaller 12AX7 tubes.

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Thanks to all who contributed to answering my questions. I re-tubed my rig and pulled the chassis to take some pics to put all of this to bed.

 

Oops..... if someone could tell me how to post pictures within a message, I could then show them [blush] :)

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First you have to upload your pictures to a picture hosting site. I use www.tinypic.com because I'm cheap and it's free!

 

To upload your picture select "browse" and tell it what folder you have your picture stored in. It will open the folder and then you need to select the filename of your picture like ...EpiExplorer.jpg. Click on this and it will load into the tinypic applet. fill out any other info like subject ( i just call it stuff or guitar or whatever I imagine). Now comes the hard part part...what size do you want the picture to be? they offer recommendations but pay no attention to those. For this forum or any other select the 320 x ??? size as it will be just about the right size to see without being either too small or too large. the applet WILL NOT affect your file. Instead, it will make a copy of your file and resize it according to your selection.

 

When you've made your selection just click on the "upload" button and the applet will do the rest. Give it a few seconds or maybe a minute or two depending on your connection speed.

 

Once your picture is uploaded the site will give you a selection of 4 different urls and each is listed under a specific purpose like email, posting at a forum, etc. I typically select the one for posting in forums as the url will include the html codes typically used in a forum. Simply copy the whole url with html tags and then come here and open a reply box or start a new thread and paste the string in the box. If you want, you can hit the preview button to make sure your picture is going to post properly. If it shows in the preview then just hit post and you've posted a picture.

 

Hopefully this is kind of what it looks like but I will put it in brackets so as not to confuse the forum as it will try to translate it:

 

{img]http:www.tinypic.com/jwbfff PIYG087dfg883rr5/htm[/img}

 

Good! This works. the first and final bracket in the string are INCORRECT because I made them that way. the first img ends with the proper bracket and the img at the end of the string begins with proper bracket. These should replace the these brackets {}. funny as it might sound the shape of the brackets are more important than what you put between them. The computer pays attention to EVERY little detail.

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