5F6A Bassman voltage check

[Jonny toetags]

I'm using the Classictone 40-18029 which has a 610 or 710 tap.
I'm using a TungSol GZ34

With the 610 tap, I was getting a voltage of about 375V and with my adjustable bias, could only get the tubes to about 16mA.
With the 710 tap, I'm getting 475V and my bias starts at 22mA and I still have the full range of the pot to raise it.

Is it safe to run it at 475V?
Or should I just leave it at 375V and change my resistor in my adjustable bias to allow more range?

[Tom Phillips]

Given those two choices I would try both and then decide which one you like best based on your playing style and preferences and the response you get from the amp. If I was starting from scratch I would have shot for 450V max but 475V is within an acceptable range. Note that 475 @ 22mA is only 10.5 W dissipation and the voltage will come down when you adjust for higher dissipation. I suggest that you start with the 475V and increase the current and you may very well find a very sweet spot setting.

[soundmasterg]

Also, do you have the rest of the tubes in circuit or just the GZ34? After all the tubes are in circuit and it is biased up as it should be, you will find your B+ is likely around 430V-440V. I agree with the 475V tap.

Greg

[Chuck H]

Also, do you have the rest of the tubes in circuit or just the GZ34? After all the tubes are in circuit and it is biased up as it should be, you will find your B+ is likely around 430V-440V. I agree with the 475V tap.

Greg

Well he IS reading bias current

[Chuck H]

Running a quick simulation on PSUD2, increasing your current from 22mA (per tube I assumed, about 40% dissipation) to 38mA (per tube, about 70% dissipation at the new, lower voltage) you'll have about 460Vp. Not entirely kosher with a 5f6a but if you pop in a 5u4 tube you'll get 430Vp at 65% dissipation. Which is just about correct. I don't know why, but this is something like the fourth time I'm finding new fender replacement PT's for amps that originally used a GZ34 only give "correct" voltages with a 5u4.

[Jonny toetags]

Tom Phillips:
-I ran the amp last night for about 4-5 hours and she was sounding good, but it still felt like it was 'missing something'. It could have been the low bias, but seeing the 375V, i couldn't help but think that was low. My adjustable bias is a 25K pot with a 30K resistor. I would probably have to lower that resistor A ALOT to get the amp in a 'proper range', which again I feel isn't quite right.
My plan for today is to swap to the 710v tap and bias her hotter. Originally I was just a little worried the 475V would stress components.

soundmasterg:
-all tubes are installed. Readings were taken after the amp had been running hard for a few hrs.

Chuck H:
-does a 5U4 change the feel at all? Can i jut pop it in or do I have to change any wiring?
modern production OK or should I go NOS.

[Chuck H]

I just had an amp here set up to simulate a gz34 rectifier and 465Vp on the EL34's. I also tried a pair of 6L6's in the amp and ran them through all the paces, clean to overdriven. Sounds good, A little stiff, but in the good way that adds note definition and not the bad way that sounds too hard. Probably what "I" would want from a 5f6a. No failures. Both power tube sets are Ruby selected Shuguangs. A 5u4 will sag a little more and the lower voltage will also play it's role. The attack envelope isn't that different. It's not like plugging in a compressor or anything. The amp would be somewhere in between where it is now and what you'll hear when you change the voltage tap. So if the lower voltage seems too low and the higher voltage seems too high a 5u4 with the 710tap might be just right. NOS or otherwise, it doesn't matter. I think 5u4's are pretty affordable either way. That if anyone even makes a modern 5u4.?. I dunno. I have three that are just leftovers from a sordid past.

[DRH1958]

Lowering the bias resistor values will give you less, not more range. With the higher voltage tap, you will need higher negative bias voltage to properly bias the tubes than with lower voltage on the plates.
Not sure if/when Chuck will reply, but you can do a direct swap of rectifier tubes. Unless I'm mistaken, you can swap all 5 volt rectifier tubes without re-wiring them, at least the common ones, 5Y3, 5U4, 5V4, 5R4 and 5AR4. You just need to pay attention to max reservoir cap value, peak reverse(inverse) voltage rating and of course, max mA rating. The 5U4GB actually supplies a little more amperage than the GZ34/5AR4, so you should be good to go. Expect about 20-30 volts less than with the GZ34. I like NOS. I'm not a fan of newer production tubes at all. Look for balanced outputs from the tube as you want both voltages to be the same or as close as possible for the full wave rectifier output.
Personally, I think having the higher voltage tap with the 5U4 will be a good combination.

Edit: looks as though Chuck and I were responding at the same time.

[Jonny toetags]

DRH1958:
-Yeah...lower, higher....to this day i still get my wording wrong when it comes to resistors
What you wrote is what I meant.

I'm going to put the amps (I built 2) through their paces today with the 710 tap and the GZ rectifier that is in there and see.
I might order the 5U4 either way to try. I order through Tubestore, so I have the choice of new production or NOS.

[DRH1958]

DRH1958:
-Yeah...lower, higher....to this day i still get my wording wrong when it comes to resistors
What you wrote is what I meant.

I'm with you there on that one. "Turn left, no I mean the OTHER left, turn right."

[Chuck H]

I'm with you there on that one. "Turn left, no I mean the OTHER left, turn right."

Turn left?
Right.
Right?
I mean correct.
It's right then?
It's left.
Turn left?
Yes! Left! Right is wrong!

[g1]

I don't know why, but this is something like the fourth time I'm finding new fender replacement PT's for amps that originally used a GZ34 only give "correct" voltages with a 5u4.

Is this even if you account for vintage line voltages?

[Chuck H]

This isn't on my own bench. This is stuff I've encountered here in the last year or so. I've considered the vintage voltage thing. But for that to be the case these transformer cloners would have to be correcting filament voltage and then duplicating what a vintage amp HV rail would be with modern AC mains.?. Why would they do that? Just adjust the primary for 120 or 125 (call it 122.5 and keep it in spec either way!). There's definitely some confusion. Probably more so in ideology than how to make a transformer

[g1]

for that to be the case these transformer cloners would have to be correcting filament voltage and then duplicating what a vintage amp HV rail would be with modern AC mains.?. Why would they do that?

No idea why, but I get the impression that is what we are getting. Maybe they are just using turns ratio?
I still see modern production x-formers marked "115V" or "117V" .
The Hammonds that now have the extra "125V" tap are a fairly recent update, at least they seem to be finally acknowledging that it is an issue.

[Jonny toetags]

So I've put probably 6-7 hours on these amps and they are sounding pretty sweet (biased at ~35mA). volume between 5 and 10 the entire time.
Now I'm debating if it's even worth trying the 5U4's.