Questions about building AB763 out of Bogen amp...

[chikov]

Thank you Martin for all your help! I just discovered that I have 470VDC on my Reverb 12AT7 tube! Oh no! I made a mistake in my reverb injection wiring. At first, I missed 500pF cap that AB763 has on the grid wire of its 12at7. Second, the Bassman schematic has 220k resistor past coupling cap out of plate of V1b tube, but the AB763 does not have it at all... should i try to do away with 220k resistor?
pS. I blew another fuse - I need to get to the bottom of this

[chikov]

Small update: I removed the resistor in front of the screen resistors and so far no fuse problems. However, I still have 471V on the plates of the Reverb tube. Does it mean that it is normal, since the both plates tight together and it is configured as an output tube? This is my last 12AT7 and I do not want to ruin it.
Please help.
PS. I just found voltage chart for AB 763 amp, and on the plates of the preamp tubes it is shown about 180V and I have 280V, and on the Reverb tube it is 410, and I have 470. On the 6L6 on Bassman AA864 should be 420V and I have 470VDC. What is the best way to reduce voltages?

[martin manning]

Do you have space to add a series R and a cap to ground between the screen node and the reverb driver tube's plates? Try an 18k resistor and a 22uF 450V cap.

[chikov]

Hmm, are you talking about the other lead of the Reverb transformer TR4? The one that goes via letter B to the screen node? Yes, i can do that.
BTW, i replaced other PS resistors between nodes B,C,D with 10k resistors like on AB763. It dropped the voltage on the preamp tubes to 220V. The amp sounds good, BUT i could hardly hear reverb, after I installed 500pf on the grid of the reverb Output tube... Any idea how i can get more reverb in my signal?

[martin manning]

Hmm, are you talking about the other lead of the Reverb transformer TR4? The one that goes via letter B to the screen node? Yes, i can do that.

Yes, correct. Play with the series resistor to get the voltage you want.

BTW, i replaced other PS resistors between nodes B,C,D with 10k resistors like on AB763. It dropped the voltage on the preamp tubes to 220V. The amp sounds good, BUT i could hardly hear reverb, after I installed 500pf on the grid of the reverb Output tube... Any idea how i can get more reverb in my signal?

Not sure what you mean there... the reverb pan's output goes directly to the grid of the reverb recovery stage. There's a 500p at the grid of the reverb driver (12AT7), though.

[chikov]

Thank you Martin! Yes, I had 20uf x 450V cap and 12.5k 2watt resistor and I connected them the way you told me - the DC voltage dropped to 425V! I also had a cold solder on my newly installed 500pf cap. After I restored it, I have enough reverb to be happy (although my Blues Jr has much more reverb, but I never crank it past 6). It still has a not-so-loud low freq hum, but it is not lead dress problem... May be just an old transformer hum (the OT and PT are about 50 yo and the choke about 70 yo).
The amp sounds good. It does not have much of a head room - goes into overdrive after 5 on the volume knob... May be i can play more with bias?
It is loud - GBE strings just spank my ears. May be it is because it does not have rectifier tube, but just the two diodes... I think I like the rec tubes more. my hi frequency switch does not make much difference. the deep switch is noticeable only when the amp is cranked loud...
however, I am very happy that my hybrid amp works! I will go back to my garage and now try to play through my pedals.

[chikov]

Hmmm, I might need some help... This amp worked ok for a couple of months, but recently one of the output tubes (6L6) failed (shorted). I ordered two more , but the new tube in the same socket blew my fuse. I tested it and it did not pass the quality test on my tube tester. I exchanged it with another good tube, tested my negative grid voltage = -46 VDC (no problem there). The plate voltage a little too high = 489 VDC. However, it worked there for a while.
Well, today I turned it on again and there was no guitar sound coming out, but a couple of crackling noises. I looked at my front ( the troubled) tube and saw a blue glow around one of the plates. Do you know what that blue glow means and what might be causing it?
thanks,

[pdf64]

A faint blue glow isn't a problem, see http://www.kcanostubes.com/content/blue-glow-tubes
Does it stop if the amp is operated without the LTP tube in place?
What cathode currents on the power tubes? ie voltage across 1 ohm cathode resistors at idle.

[chikov]

A faint blue glow isn't a problem, see http://www.kcanostubes.com/content/blue-glow-tubes
Does it stop if the amp is operated without the LTP tube in place?
What cathode currents on the power tubes? ie voltage across 1 ohm cathode resistors at idle.

I will check voltages tonght, after I get home from work. I do have a question, what does LTP stand for? Do you mean "phase inverter" tube?

[Phil_S]

LTP=Long Tail Pair phase inverter.

[chikov]

LTP=Long Tail Pair phase inverter.

Thanks Phil. I recon, it is one of those questions that you are aloowed to ask only once on TAG ))
Back to my amp. I have a good news. Today the amp started working again. I checked the resistance on my cathode ones and one was 1.2 and another 0.7. I replaced the smaller value one with 1.3 ohm one. Surprizingly, even though the tubes at the moment are different brands, the voltage across was .068 V DC. The plate voltage is a little high at 493 VDC. But, Leo Fender used to use high voltages, I heard. I hope it s ok...

[chikov]

I have a small question... tonght, after I played my amp for a while and quickly turned it off/on, my 1.5 amp fuse blew. I replaced it with another one and it started working, but i can still hear my guitar going through even with main volume on zero. Can it be my V1a and V1b shorting between the triodes? I never had that happen before.. the V1 does seem a little microphonic..
Update... I tested my 12ax7 on the tube tester - all good. I tried another vintage 12ax7 in its place - I can still hear the guitar when the volume is off (quiet but still there). So strange... I do not know if it helps, but another strange thing is when I turn my power switch on ( but the Standby off) I hear a quiet humm...

[chikov]

I figured, by the lack of the replies, that my question was too ambiguous. So I decided to figure it out by myself. The Bassman cirquit is very solid, but one big weakness I had was my own customized Bias circuit. I realized, that Voltage Doubler circuit does not work with regular Fender bias wiring, so I decided to study up and re-do it. This is the circuit I decided to use:

[IMG:480:640]http://i409.photobucket.com/albums/pp171/alexjohnne/IMG_0240_zpskwheol5a.png[/img]

It worked perfectly. I got, I think -58V Bias voltage on my grids. That took me to about 48mAmps plate current and it brought up my plate voltage up to 503 VDC. That makes my Plate Dissipation about 23 Watts for 6L6GC tubes, which is reasonable. I am wondering if 503 Volts on my plates is too much and how shoul I go about lowering that voltage? May be by lowering my Voltage doubler Capacitors' value? any thoughts anyone?

[Phil_S]

I am wondering if 503 Volts on my plates is too much and how shoul I go about lowering that voltage? May be by lowering my Voltage doubler Capacitors' value? any thoughts anyone?

The answer is that it depends on the rest of the circuit and the voltages. Would you like to post a voltage chart, even betterm a full schematic with voltages?

I"m not the most experienced person here. Far from it. I don't see a doubler there. That looks like a FWB with a cap across it. I don't think I've seen this before. What happens to the voltage when you disconnect that cap?

Please avoid photobucket. There is only an annoying window through to photobucket and the image is about the same size. I can barely see that diagram and I'm using a 21" monitor. Post any pictures or documents directly to the forum so we can enlarge them with a simple click.

The most expedient way to drop about 40-50V is to use a 5U4GB tube rectifier if you've got a 5V @ 3A winding and a place on the chassis for the socket. There are other ways. You will find discussions here if you search.

[sluckey]

That looks like a FWB with a cap across it. I don't think I've seen this before. What happens to the voltage when you disconnect that cap?

That cap is the first cap (reservoir) in the B+ rail. It's just drawn that way for simplicity.