ampeg b12xt driving me crazy

[unclerny]

Hi Forum, haven't posted for a while so I must be getting better at this.

I've been pulling my hair out on an Ampeg b12xt for some time now.

It doesn't have much volume but has all the right voltages, new tubes and power caps, NOS 7199 (tried two), swapped the OT with a fender one just long enough to prove that wasn't the problem, didn't really think it was but...  Oh, tried a different speaker too.

Both channels put out about the same signal and at the ext amp jack both are clean and full, can't speak as to how much signal is there but it looks and sounds about right to me.  I've tried everything I can think of to get it as loud as the other b12 we have on the floor and it's just not happening.

Any fresh ideas would be most helpful.

Thanks, Tim
UEE

[Willabe]

Both channels put out about the same signal and at the ext amp jack both are clean and full, can't speak as to how much signal is there but it looks and sounds about right to me.  I've tried everything I can think of to get it as loud as the other b12 we have on the floor and it's just not happening.

Hi unclerny,

Are you saying that when you plug the speaker into the "normal" output jack you have volume loss, but when you plug the speaker into the ext. speaker jack the volume output and tone are normal?

If so it sounds like there's something wrong with the speaker jack/s and/or the way there wired.            Could be the switch on the normal speaker jack or maybe a ground issue?


                   Brad           


                         

[unclerny]

Hi Brad, this amp has an ext amp jack and an ext spkr jack.  from the ext amp it sounds fine.  I don't have the volume from the power amp that I think it should have.  Not really distorted although more than the pure clean tone from the ext amp out.  It's about half the output I'd expect.

What about wrong speaker ohm load?  That's something I haven't thought about.  I'll get back.

Tim

[Willabe]

this amp has an ext amp jack and an ext spkr jack.  from the ext amp it sounds fine.  I don't have the volume from the power amp that I think it should have. 

So the ext. amp jack is a preamp out, and the preamp out sounds fine? Then it should be somewhere in the power amp. Are you sure the OP tube voltages are right? Could you post them so some of the guys can look at them?


                Brad           
 

[Willabe]

I just looked at the schemo, I should have done that 1'st.

I see the pre out jack now. It would be helpfull if you could post the OP tube --and-- the PI volatges.

Some of the guys here can spot a problem in the tube voltages like a hawk can spot a snake in the grass at 550 ft!             


                 Brad     


                   

[unclerny]

Hi Willabe,
B+ 525      that sounds about right considering the schematic calls for 117vac and my service is 120vac

I use Weber Bias Right and their calculator as a base guide.  That means 40m.A. on the tubes.  That sets the negative bias at -55vdc.  The amp runs stable and for all intensive purposes it appears to be working fine.

I also set the neg. bias at -44vdc as per the schematic and it runs stable there too.  Not any louder, tubes are hot enough to toast bread so I'm sure it would lead to short life.

Back to my post

What about wrong speaker ohm load?  That's something I haven't thought about.  I'll get back.

I went in on Sunday and checked the speakers.  One is a replacement JBL, the other original.  Someone wired them in '72, because I have the receipt showing the JBL purchase, in parallel and not series.  The schematic clearly shows two 8 ohm drivers in series.  Huge difference in volume.  That was the problem.  Can you believe someone had used it that way for all those years.  I bet who ever owned it thought the guy who put in the speakers had royally wrecked his amp!!

I was testing it with my shop 4-12 cab and bridging the cap inside to turn it on.  Yes, the four pin speaker cord is still original.  Anyway, that cab is 8 ohms.  I'm setting there thinking "damn the amp is almost there, what am I missing?"  Rewired the cab JBLs, plugged it in as designed and problem solved.

This forum always helps me, I love it.  Besides help from others it also makes me think away from the problem.

Thank you Thank you All
Tim
UEE

[Willabe]

Glad to hear you got it going at full power again.

RCA tube book says 7027A, 600dcv max plate and 500dcv screen grid. Ampeg schemo says 475 plate 450 screen. If you have 525dcv on plate, where's your screen voltage at? 525 on a 600dcv plate should be more than fine, but if the screen is at 500dcv that would make me worrie. You did add screen grid resistors?


                 Brad            

[unclerny]

Hi Willabe thanks for getting back.

we put 6L6GC in it for the obvious cost/availability reasons.

No I didn't modify the circuit in any way.  Trying to stay as stock as possible for resale.  Now that I look at the schematic I see what you're talking about.  Some 1k's in there would make me much more comfortable.  I'll see if John wants them put in.  Hard to believe in an amp this powerful they didn't have them in there.

Tim

[Willabe]

we put 6L6GC in it for the obvious cost/availability reasons.

Hi Tim,     OK,  but now......

....    6L6GC max plate dcv RCA book value say's 500dcv which is 100dcv's max less then 7025A, and your at already at 525dcv on the plates. Thats a gonna push them to maybe over their ability. Plus/but the screen grid max on 6L6GC is only 450dcv. 50dcv's less than 7027A and you have no screen grid resistors/screen grid stoppers. The screen grid voltage won't take as much abuse as the plate. What is you're screen grid dcv? This is what I'd be concerned about, just my $.02.

No I didn't modify the circuit in any way.  Trying to stay as stock as possible for resale.  Now that I look at the schematic I see what you're talking about.  Some 1k's in there would make me much more comfortable.

You don't have to go as high as 1K, 470R @ 2w will probly give enough safety.

As far as keeping the amp stock, you put the SG resistor across pin #6 (B+ feed) to pin # 4, ---- BUT ONLY ON 6L6 - type - tube !!! 7025A has different pin out then 6L6 type tubes. If you go back to using 7027A tube then you run the SG stopper from pin # 4 (or pin # 2) back to the B+ screen voltage feed/node. Both ways easly removed, put back to "stock".  

But your still have too high of a screen grid voltage using 6L6GC in this amp IMO.

Maybe I'm wrong, but I'm sure some of the other guys here will straighten this/me out if I am.

These older amp's were made/built to "only " be turned up to 4, maybe 5 on the volume, when "they" were pumping out high Q tubes! Plus "if" a tube went down the replacement $$ was no big deal.  


         Brad                    

[PRR]

6L6GC and 7027 are, for all intents and purposes the same tube except for the extra G2 connection. There's another version, same except plate-cap for extreme TV voltages.

The official story is that G2 gets better cooling from the extra pin. I suspect the real reason was to charge different prices for the two "not interchangeable" tube-types. i.e. the "450V" rating on 6L6GC is marketing, not engineering.

Clean your spare G2 pin. Small amps to 40W/pair, use 6L6GC (any recent "6L6" type). Over 40W, use 6550.

FWIW, compared to hamburgers or gasoline, tubes today are significantly cheaper than in 1972.