Showman> 6v6 pair load question

[deci belle]

I acquired a '66 sgl Showman/115 8Ω cab and have gone through the filter section, the rectifier and the preamp/PI. I added an extra filter stage and brought the normal channel's voltage down for the 5f6a/extra gain stage mod. But …four 6L6s are too loud— so I'm going to use 6v6s.

My main concerns right now in changing from 6L6 to 6v6 tubes are my 8Ω OT's impedance compatibility, the bias feed resistor's value and NFB considerations.

Originally I was going to use four 6v6s, but If I'm understanding correctly from studying the related threads, two 6v6s have a comparable OT load requirement as a quad of 6L6 tubes, so I should use a pair of 6v6s with the same 8Ω speaker I'm using now and still have a happy OT.

Will someone please confirm that for me?

As for setting up the bias supply, I have –60v at the bias supply R, –50v at the center tab on the bias pot and –41v at pin 5. I'm running 25.8ma with the 6L6s.

My plates have 465v, so I want to start the 6v6s at 16ma (7.4w/ 50%). I'm not sure which way to go in R value to see about –20v (DR grid value I saw on a schematic). Any suggestions on that value?

I want to pull the pwr tubes to dial in the bias supply R, is that the right way to proceed?

I'm not intending to sound like a DR, I just want to lower the volume without having to replace the original OT and d130f.

The threads I found compared two or four 6v6s to a pair of 6L6s, so I'm confused about the NFB issues that were addressed, as I plan to go from four 6L6s to a pair of 6v6s.

If anyone would help guide me in a viable direction, I would appreciate it!

[ThermionicScott]

By "too loud", do you mean that it doesn't break up soon enough? Not a surprise -- that was the intent of these amps.

If nothing else, you could throw in a quad of JJ 6V6s and bias them hot. The "too-low" impedance will just reduce power output safely.

Not sure where you're getting -20V. All the old DR schematics show -35V.

- Scott

[olddawg]

I acquired a '66 sgl Showman/115 8Ω cab and have gone through the filter section, the rectifier and the preamp/PI. I added an extra filter stage and brought the normal channel's voltage down for the 5f6a/extra gain stage mod. But …four 6L6s are too loud— so I'm going to use 6v6s.

My main concerns right now in changing from 6L6 to 6v6 tubes are my 8Ω OT's impedance compatibility, the bias feed resistor's value and NFB considerations.

Originally I was going to use four 6v6s, but If I'm understanding correctly from studying the related threads, two 6v6s have a comparable OT load requirement as a quad of 6L6 tubes, so I should use a pair of 6v6s with the same 8Ω speaker I'm using now and still have a happy OT.

Will someone please confirm that for me?

As for setting up the bias supply, I have –60v at the bias supply R, –50v at the center tab on the bias pot and –41v at pin 5. I'm running 25.8ma with the 6L6s.

My plates have 465v, so I want to start the 6v6s at 16ma (7.4w/ 50%). I'm not sure which way to go in R value to see about –20v (DR grid value I saw on a schematic). Any suggestions on that value?

I want to pull the pwr tubes to dial in the bias supply R, is that the right way to proceed?

I'm not intending to sound like a DR, I just want to lower the volume without having to replace the original OT and d130f.

The threads I found compared two or four 6v6s to a pair of 6L6s, so I'm confused about the NFB issues that were addressed, as I plan to go from four 6L6s to a pair of 6v6s.

If anyone would help guide me in a viable direction, I would appreciate it!

Have you considered just pulling 2 of the 6L6 tubes and adjusting your load with an 8 ohm resistor?

[deci belle]

Hi Scott! Ok, this is a start! I've heard so many horror stories about using 6v6s over 420~450v… this is rather scary because I've got 465v on my plates.

So you are saying I can use the quad of JJ 6v6 because the impedance differential will work out in this instance— Is it advisable to get a 16Ω speaker, even though the 8Ω will work ok?

Yeah, it won't break up— I like a clean vibrato channel, but the tweed/gain normal channel isn't very dirty at all except when it hurts!

That –35v sounds right, thank you! Would you please tell me where I should measure that, Scott? At pin#5, the bias pot or at the bias supply resistor?

How hot do you suggest? I'm planning to set the bias initially at 16ma.

Thanks so much, Scott!

Mar'

[deci belle]

Hi oldawg! Oh yes, my first thought was to pull two of the tubes~ But I don't know why or what kind of 8Ω resistor would work, or where I would put it though… I've only been working on amps for a year! So that's the cause of my ignorance and trepidation.

At this point (since I already bought tubes) I believe I'd enjoy 6v6s, if I can keep them alive in this amp, but would you mind clarifying the correct way to adjust my load with an 8Ω resistor in this instance? That's something I should know too!

Thanks olddawg!!

[ThermionicScott]

Hi Scott! Ok, this is a start! I've heard so many horror stories about using 6v6s over 420~450v… this is rather scary because I've got 465v on my plates.

So you are saying I can use the quad of JJ 6v6 because the impedance differential will work out in this instance— Is it advisable to get a 16Ω speaker, even though the 8Ω will work ok?

Yeah, it won't break up— I like a clean vibrato channel, but the tweed/gain normal channel isn't very dirty at all except when it hurts!

That –35v sounds right, thank you! Would you please tell me where I should measure that, Scott? At pin#5, the bias pot or at the bias supply resistor?

How hot do you suggest? I'm planning to set the bias initially at 16ma.

Thanks so much, Scott!

Mar'

465V is pretty high, and I wouldn't recommend any other 6V6 but JJ -- they're designed for high voltage and dissipation. The impedance mismatch reduces power because the load line is steeper -- the tube plates can't get the full voltage swing. This also prevents the screens from drawing as much current at full power, which is why it's safer for the tubes. All that said, this may still end up a loud amp, so you might consider adding power-scaling, or building a smaller amp with some of the Showman's features.

[deci belle]

Ok, Scott! The JJs are the ones I've ordered. I'm beginning to grasp the load line thing, too— from the Wizard's site. So what you told me helps. Do I measure –35v at the tube?

Yeah, it will be loud!

Thank you, Scott!

[olddawg]

Hi oldawg! Oh yes, my first thought was to pull two of the tubes~ But I don't know why or what kind of 8Ω resistor would work, or where I would put it though… I've only been working on amps for a year! So that's the cause of my ignorance and trepidation.

At this point (since I already bought tubes) I believe I'd enjoy 6v6s, if I can keep them alive in this amp, but would you mind clarifying the correct way to adjust my load with an 8Ω resistor in this instance? That's something I should know too!

Thanks olddawg!!

You just need to pull the two outer or inner 6L6 tubes and put an 8 ohm ceramic 20 watt resistor in series with the positive terminal of the 8 ohm speaker (you can even mount it inside the amp) to make a 16 ohm load. The amp is probably tough enough to just pull the tubes, but matching the load is better. And....in your case adding the resistor will also drop some more of the power. It's a lot cheaper and easier than buying a bunch of tubes, lowering the B+, changing the bias, and such. It's a very common practice and an 8 ohm / 20 watt resistor is less than $10.

[ThermionicScott]

The bias voltage is usually measured before the 220k bias-feed resistors. It'll be a little less at the tubes themselves. If you go with 6V6's you'll need more than -35V at those voltages, maybe -45V.

-Scott

[deci belle]

Great olddawg! I've always heard of changing the speaker— I'll try this for sure before the 6L6s come. Thank you so much for this tip! —Mar'

[deci belle]

Ok, Scott— that's what I needed to hear. I'll shoot for about –40v and see what my ma range is at the cathodes, before going higher.

I want to set the bias resistor with the power tubes out— is this standard procedure?

[ThermionicScott]

I want to set the bias resistor with the power tubes out— is this standard procedure?

No, but it's a good idea! Some of the RCA example circuits had an instruction to remove the power tubes and set the bias voltage as negative as possible (coldest bias) before re-installing them.

- Scott

[deci belle]

Right on. Thanks, man!

[redelephant]

+1 with Olddawg.
If you still wanna use 6V6, why not go to cathode bias? That'll give some voltage drop over the cathode resistor, whcih gives more proper plate voltage for those tubes.

[deci belle]

I barely have my head wrapped around the fixed bias the amp came with, redelephant!! hahha! Someday soon I'll be ready to learn about cathode biased circuits and follow your suggestion I will find a smaller amp, though.

The Showman kinda fell into my lap completely original, but disassembled— I even had to glue the old tolex back on the head and re-attach the grill-cloth…so it's my designated fat pencil test-mule! Hopefully I'll score an old dual 6v6 cathode bias amp from somebody eventually and experiment with those cathode resistors you mentioned!