Questions on a build of a modified 1968 Ampeg B-15 N.

[J Luth]

This amp is currently built but looking to get info on tube rectification and a few other things. It actually works quite well but I don't want to blow up any good rectifier tubes.

Amp is only one channel with a 6SL7 pre amp and a 6SL7 phase inverter but it has 4, 7581A or can use 4, 6L6GC power tubes. The required transformers are not a problem, already have some that work.

First question does anyone know if there is a single rectifier tube that can safely handle this setup? Dual rectifiers would probably the way to go but space will not allow that easily but may be possible. Some that I have here are GZ34 probably a big no on that, 5U4, 5R4, 5931 and JAN-Chatham 5R4WGA. All with the exception of the GZ34 are vintage U.S.A. made tubes.

It also has SS rectification but want to switch between SS and tube.

Next question, if there isn’t a single tube rectifier that can handle it, is there a circuit that can be installed that can be used to disable 2 of the power tubes when using the tube rectifier?

Other changes are, no hum pot, 100R resistors used instead, bias changed to fixed using 10K pot and resistors like a Fender amp. Power supply changes are first reservoir cap is changed to 2, 80UF in series with 2, 220K balance resistors, all other caps are 40 UF/ 500V

[g1]

Is it going to be B-15 power output or something else? It depends on what you are doing with transformers.
If all is same as B-15, just with 4 power tubes instead of 2, I don't see why a single rectifier tube would be a problem.
Twin 5F8 and 5F8-A had four 5881's and a 5AR4, running around 400V B+.

[J Luth]

g1 Not sure what you mean by Is it going to be B-15 power output or something else?
The circuit is basically the B 15 schematic I posted but with only 1 channel and 2 more power tubes added. The output transformer is out of a 80's, 90's or later amp, don't remember what amp it was right now but can find out, that was running 4 6l6GC. I have had the amp running with a 5U4 and 5931 for a bit but not for long intervals. When I tried it with some reissue GZ34's they looked like the a wonderful fireworks show inside the tube after about 3 or 4 minutes. I thought there might be a short in the circuit somewhere but none found. The other rectifiers don't do it.

[Chuck H]

When I tried it with some reissue GZ34's they looked like the a wonderful fireworks show inside the tube after about 3 or 4 minutes. I thought there might be a short in the circuit somewhere but none found. The other rectifiers don't do it.

Maybe tubes that aren't actually up to the vintage spec??? That does happen. EL34's are supposed to be capable of higher plate voltages than any "modern" EL34 will handle. Yet the data sheets just regurgitate the vintage specs verbatim as if that weren't the case.

A 5ar4 should be up to the task, as g1 pointed out. But THOSE 5ar4's may not ACTUALLY be.?. In any case their current capacity should be up to the job. It's probably the peak inverse voltage that isn't. Tube amp HV peaks on the OT primary can be huge. You might try putting a diode in series with each rectifier tube plate to increase the PIV capacity for the rectifier circuit and protect the tube. You'll still get the rectifier tube sound. The typical 1n4007 would probably do. There are burlier diodes too if you need to order some anyway.

[Helmholtz]

For reference:

A 100W amp draws around 500mA at full power before clipping.

Rectifier current limits:

GZ34/5AR4: 250mA
5U4GB: 275mA
5R4: 250mA
5931: 300mA
5AT4: 800mA

[Chuck H]

A 100W amp draws around 500mA at full power before clipping.

Well that would be more than I thought. Thank you for the correction.

So what's up with the Fender Twin amps? I'll guess them at 80 watts? Did they blow up rectifiers all the time?

[Helmholtz]

Well that would be more than I thought. Thank you for the correction.

So what's up with the Fender Twin amps? I'll guess them at 80 watts? Did they blow up rectifiers all the time?

Dunno.
One might get away with only one rectifier tube if the PT sag limits power output to maybe 60W.

[Chuck H]

Dunno.
One night get away with only one rectifier tube if the PT sag limits power output to maybe 60W.

I think that's a pretty good speculation.

[g1]

The high power tweed Twins are supposed to put out 80W. I have never tested myself so can not guarantee that. Fender spec'd them for 85W.

g1 Not sure what you mean by Is it going to be B-15 power output or something else?

It's all about the power transformer, you have not mentioned what it is.

[Helmholtz]

Well some amps were "short-lived" .

[g1]

I don't want to get side tracked with the tweed Twin, it was just an example of a real schematic showing 4 power tubes on a single rectifier tube.

The B-15N is a 25W amp.(edit: The 'NF' version in the schematic from the first post is rated 30W). Even with double the PT power capability, we are still only in the 60W range.

J Luth , you said the amp is working with a SS rectifier. What is the measured power output of the amp?

[Helmholtz]

Just measure supply DC current at full output.

[J Luth]

A bit more.
The 5ar4 only showed issues with a 1K, 1mV signal at full power, the other rectifiers I have tried, 5u4 and 5931 have worked fine so far. I have not tried the amp for any extended time yet.

Probably going on hold for a bit now seeing as how my old dumb A$$ decided to give myself a birthday present yesterday by breaking my big toe on my left foot. They don't want me walking on it much especially going up and down my narrow basement stairs. Am I going to do as they want? Probably not for very long.
This getting old stuff sure ain't a lot of fun.

[J Luth]

g1 when I can get to it I will see if I can get the info on the power trans. But probably be a while
See post #13

[Chuck H]

A bit more.
The 5ar4 only showed issues with a 1K, 1mV signal at full power,..

1mV ??? Is this a typo?

Probably going on hold for a bit now seeing as how my old dumb A$$ decided to give myself a birthday present yesterday by breaking my big toe on my left foot.

The BIG TOE!!? I've never heard of anyone doing that. Must hurt like hell. Well wishes and a speedy recovery.