Tweed Deluxe Build - Consolidated Discussion

[TheTinMan]

I'm going to through the process of specifying and building my first Tweed Deluxe. My goal is as close to "vintage" as possible but with 1 or more switches for other options.

Thought it would be useful to tie related threads together:

Transformers and Speaker

Switch for HV AC Taps

Based on these discussions and other research, I've decided on a Weber 12A125-A speaker and these ClassicTone transformers:
PT 40-18078 with 330-0-330 and 355-0-355 secondary taps, rated for 100 ma
OT 40-18022 with 8K primary and 8 ohm secondary

Switching the high voltage secondaries raised several safety concerns. R.G. a tube/solid state rectifier switch to have higher voltage option instead of switching HV secondaries. Simpler, safer = reasonable. My thought is to use a double-pole On-Off-On switch with one pole selecting the rectification and the other pole selecting a higher power tube cathode resistor for the higher voltage solid state setting. Off would be standby. I don't care about a standby switch but an intermediate Off state seems like a good idea here.

POWER SWITCHING
Which would be more useful/beneficial in a 5E3 circuit:
A) tube/SS switching with approximately 50 volt B+ jump
B) low power switch adding 330 ohm resistor under power tube cathode bypass cap(s). See right side of page 3 here: http://sluckeyamps.com/misc/Amp_Scrapbook.pdf
C) Cathode bias // fixed bias switch

Need to do something with Ground switch that doesn't involve bringing sensitive signals into AC and current heavy environment at that end of the chassis. "None of the above" may be the right answer too, so,please share your thoughts.

INPUT JACKS
I'm also leaning toward only two input jacks Hi/Lo with a switch for Bright-Both-Normal. That would leave the fourth input jack hole available for a switch or pot. However, I'll keep all four jacks if different manual jumpers actually produce different results. Not clear on that so please advise me.

MASTER VOLUME
A pre-PI master replacing the concertina splitter's 1 meg grid return resistor with a pot. Possibly a 500K pot on top of a 500k resistor on the theory that turning Master down below half might not be useful. Thoughts?

There are a few other minor ideas I'd like to experiment with, but these are significant circuit mods. There's not much room in a Tweed Deluxe chassis, so I need to design layout to accommodate possible mods.

Thanks in advance for your thoughts!

Chip

[Leo_Gnardo]

POWER SWITCHING
Which would be more useful/beneficial in a 5E3 circuit:
A) tube/SS switching with approximately 50 volt B+ jump
B) low power switch adding 330 ohm resistor under power tube cathode bypass cap(s). See right side of page 3 here: http://sluckeyamps.com/misc/Amp_Scrapbook.pdf
C) Cathode bias // fixed bias switch

Need to do something with Ground switch that doesn't involve bringing sensitive signals into AC and current heavy environment at that end of the chassis. "None of the above" may be the right answer too, so,please share your thoughts.

Might I suggest, use that "ground switch" location for a normal/triode choice. IMHO that would be enough power section choice.

If you must do more, I can tell you it's hardly worth messing with the fixed bias/cathode bias switch. I've put in a few, and very hard to hear the difference though your scope & meter will tell you 2x the power fixed bias compared to cathode bias. I don't do those anymore.

[ThermionicScott]

Might I suggest, use that "ground switch" location for a normal/triode choice. IMHO that would be enough power section choice.

If you must do more, I can tell you it's hardly worth messing with the fixed bias/cathode bias switch. I've put in a few, and very hard to hear the difference though your scope & meter will tell you 2x the power fixed bias compared to cathode bias. I don't do those anymore.

Leo, are you a fan of triode mode? I haven't gotten around to trying it myself, but I've read of people that were similarly disappointed by the sound or the degree of power reduction. Pretty easy way to make the measured power output go down, though!

I think if I were after nice output tube distortion at a much more polite volume, I'd skip everything I've tried so far and go straight for the VVR. Like you say about fixed/cathode-bias, and I've observed about switching PT secondaries, there's just enough difference to be noticeable, but nothing earth-shaking.

[TheTinMan]

Key question - does lower decibel power tube distortion make sense in the 5E3 circuit?

My impression is that these amps aren't all that loud and start breaking up quickly.

I don't play in a band but live where making some noise isn't a problem.

Chip

[Leo_Gnardo]

Leo, are you a fan of triode mode? I haven't gotten around to trying it myself, but I've read of people that were similarly disappointed by the sound/amount of power reduction. Pretty easy way to make the measured power output go down, though!

I think if I were after nice output tube distortion at a much more polite volume, I'd skip everything I've tried so far and go straight for the VVR. Like you say about fixed/cathode-bias, and I've observed about switching PT secondaries, there's just enough difference to be noticeable, but nothing earth-shaking.

I've had quite a number of customers asking for a way to knock down amp power in an obvious way - triode definitely does this. Seems in the last 20 years or so there's been a need to keep volumes down especially in club/bar performances and triode mode does the trick for them. Cuts the power down to about one third, rolls off the top frequencies quite noticeably, and also reduces power amp sensitivity so that you have to crank up the pre a point or two to compensate. More than once, a guitarist rang up to tell me "your mod saved my gig, man!" and "the rest of the band's jazzed with my new tone."
Often enough the amps are Deluxe Reverbs, old and RI, and they still get plenty loud with onset of clipping around 6 watts. Played hard, they break up just fine but don't sound annoying when pushed to and beyond the breakup zone.

If you really need to get quiet, say you're playing for the Blue Haired Ladies Garden Club Annual Luncheon and you don't want to blow their wigs off, maybe the VVR is more appropriate.

Of course I can understand, triode mode may not be everybody's cuppa tea, but it's cheap enough to try and you can undo it easily enough if you don't get along with it. The only time anyone's asked me to remove the mod, maybe two cases when they wanted to sell their collector item Fender in stock condition.

[TheTinMan]

Pre-Cathodyne Master Volume and Grid Stopper

Merlin Bledsoe writes about using a large grid stopper on a cathodyne PI to avoid nasty distortion when it is overdriven.
The Valve Wizard

Typical pre-PI master volume here increases grid stopper as voltage swing to power tubes goes down. Please comment on the attached alternative approach. With volume all the way up, it appears stock 5E3 except for 500K grid stopper. Grid stopper value goes down as volume goes down.

However, have I effectively increased the grid return resistor to 1.5K? IOW should I change the 1 meg resistor on the volume pot wiper to 500K to keep stock bias of cathodyne when volume all the way up?

Thanks,
Chip

P.S. Sorry about hand drawn schematic. No access to PC for a couple of weeks.

[Dave H]

Please comment on the attached alternative approach. With volume all the way up, it appears stock 5E3 except for 500K grid stopper. Grid stopper value goes down as volume goes down.

I don't think it will work as a master volume. The volume won't go down.

[eschertron]

I don't think it will work as a master volume. The volume won't go down.

Maybe by a few (3 or 4) dB, but nothing conspicuous. Might be worth a try, though!

[Dave H]

B) low power switch adding 330 ohm resistor under power tube cathode bypass cap(s). See right side of page 3 here: http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

That's more like a low gain switch than a low power switch. The power amp will still clip at the same power (or a little less if the PI clips first) but its voltage gain will be roughly half.

[Mick Bailey]

With the circuit you've drawn you'll have DC on the pot, so it will scratch quite badly.

[Dave H]

Maybe by a few (3 or 4) dB, but nothing conspicuous. Might be worth a try, though!

I was thinking that the bootstrapped input of the the cathodyne is high impedance compared to 500k so the attenuation would be small.

[ThermionicScott]

I've had quite a number of customers asking for a way to knock down amp power in an obvious way - triode definitely does this. Seems in the last 20 years or so there's been a need to keep volumes down especially in club/bar performances and triode mode does the trick for them. Cuts the power down to about one third, rolls off the top frequencies quite noticeably, and also reduces power amp sensitivity so that you have to crank up the pre a point or two to compensate. More than once, a guitarist rang up to tell me "your mod saved my gig, man!" and "the rest of the band's jazzed with my new tone."
Often enough the amps are Deluxe Reverbs, old and RI, and they still get plenty loud with onset of clipping around 6 watts. Played hard, they break up just fine but don't sound annoying when pushed to and beyond the breakup zone.

If you really need to get quiet, say you're playing for the Blue Haired Ladies Garden Club Annual Luncheon and you don't want to blow their wigs off, maybe the VVR is more appropriate.

Of course I can understand, triode mode may not be everybody's cuppa tea, but it's cheap enough to try and you can undo it easily enough if you don't get along with it. The only time anyone's asked me to remove the mod, maybe two cases when they wanted to sell their collector item Fender in stock condition.

That's very cool! I stand enlightened.

[eschertron]

I was thinking that the bootstrapped input of the the cathodyne is high impedance compared to 500k so the attenuation would be small.

OK. Your point may be correct, I don't have the theory background to convince myself one way or t'other. But I will assert that a 4dB range for a MV control is still small so yes, I agree with you on that!

[Dave H]

I don't have the theory background to convince myself one way or t'other. But I will assert that a 4dB range for a MV control is still small

OK It doesn't need much theory to see what the max attenuation would be. Max attenuation is with no bootstrapping and the 1M grounded, which is a 1M/(1M+0.5M) ie. 0.667 or -3.5dB. I don't trust my maths to calculate the input impedance with bootstrapping. I'll attempt an LTSpice sim.

[Dave H]

I'll attempt an LTSpice sim.

I tested it on the LTSpice schematic I have of my 'Double Deluxe'. Setting it well below clipping it gives 7.202V rms and 6.980V rms over the full sweep of the pot. That's a difference of 0.27dB.