Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Dumbmonkey on November 09, 2023, 08:42:19 am
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Hi!
I recently built a 5e3 using Rob Robinette's Optimized 5e3 layout, and I have some questions about voltages if anyone would like to chime in. I used a Hammond T290BBX for power and a T1750E for the output. The amp sounds great, but I noticed that it stays pretty clean and doesn't seem to break up as quickly as other 5e3s that I see videos for. I compared my voltages to a diagram that Rob Robinette posted, and mine are slightly higher but all comparative from what I can tell. Could the higher headroom I'm getting be attributed to the higher voltages?
Thanks for your time!
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I think it will be difficult for anyone to chime in without a schematic and the actual voltages in your amp.
/Max
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Hi and thanks for replying!
As far as I know, it's using the standard 5e3 schematic, although the layout is different
https://robrobinette.com/images/Guitar/5E3P_Build/5e3_Deluxe_Amp_Optimized_Layout.png
The voltages I have are:
B+1 387
B+2 345
B+3 270
V1 12AX7A
#1 147
#3 1.47
#6 174
#8 1.4
V2 12AX7A
#1 184
#3 1.39
#6 219
#7 15.5
#8 50
V3 & V4 6V6GT
#3 379
#4 343
#8 21
Thanks again!
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Assuming 250Ω Rk on the output tubes, I think the amp is biased pretty hot. Divide the cathode voltage with the resistor to figure out the current (Vk ÷ R = 0.×× A). Multiply that number with the plate voltage to figure out the idle dissipation.
The other voltages looks like they're in the ball park.
Also, using linear pots as most old tweeds did gives you the dirt earlier on the dial. I often prefer linear pots in my amps and then fine tune the volume with my guitars volume pot.
Hope this helps.
/Max
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Never built one of these, so disclaimer up front, but isn't V1 meant to be a 12AY7?
Also is one of the V1 plate voltages a typo? (147 & 174)
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Hi again,
Thanks for the responses. I was wondering if the bias might be too hot. I'll stick my bias probe on there later and see what is says. I'll double check the voltages on V1 I may have written them down wrong although I did split the cathodes on that tube for a lead channel mod, so could that cause a difference in the voltages? (yes, I am using a 12AX7 there, which is ok from what I hear)
I may have to increase the cathode resistor value...
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What other mods did you do? I thought you said it was standard 5E3?
You dont need a bias probe if you just calculate the dissipation manually.
/Max
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It’s a standard 5e3 with that other layout and a split cathode mod on V1.
I know I should measure bias the correct way and I will be using Rob robinettes instructions to do so this weekend, but for the record my eurotubes probe shows plate voltage at 364 and plate current at 40, which is probably high right?
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Look at the tube data sheet.
/Max
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5E3 sounds best with the 6V6s running full bore at Pmax - don't be shy - won't hurt good 6V6s.
Optimal B+ to aim for with a stock standard 5E3 is 350V, so ...
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I recently built a 5e3 ... The amp sounds great, but I noticed that it stays pretty clean and doesn't seem to break up as quickly as other 5e3s that I see videos for. ... my voltages ... are slightly higher ... Could the higher headroom I'm getting be attributed to the higher voltages?
The voltages I have are:
B+1 387
B+2 345
B+3 270
V1 12AX7A
#1 147
#3 1.47
#6 174
#8 1.4
V2 12AX7A
#1 184
#3 1.39
#6 219
#7 15.5
#8 50
V3 & V4 6V6GT
#3 379
#4 343
#8 21
My opinion is that your amp is fine, and that you might be mislead comparing amp-in-room to unknown-conditions-in-video.
I see this a lot on forums, where players are fooled by the relative-loudness of talking & distorted-amp being similar in the video. However, that's often merely the video editor normalizing volumes so that the viewer doesn't need to make major volume changes between sections when watching the playback.
A tweed Deluxe is plenty-loud when it distorts, which by design is when pushing the power tubes beyond making all the clean output power they can (15 watts?). At that point, typical lower-sensitivity Jensen-style alnico speakers would be making ≤106dB, which is quite loud. So if you think "my amp has a lot of headroom" based on relative loudness of "talking" to "Amp-dirt" then your amp is probably normal.
Maybe you're making the judgment based on the setting of your amp's Volume control compared to some setting shown in a video. Esquirefreak was right to say if your amp uses audio-taper pots and the video-amp uses linear-taper, that could account for the difference of settings. And there's no telling how your guitar's pickup-output compares to the video-guitar, etc.
Based on Robinette's diagram of typical 5E3 voltages (https://robrobinette.com/images/Guitar/5E3P_Build/5E3_Voltages.png) (and what I've seen over time), I don't think your slightly-higher voltages would cause a significant headroom increase. That said, original 1950s Deluxe amps had voltages & output tube bias-voltage creep up over time, possibly to raise output power slightly. Your amp's 21v across the cathode resistor is very close to Robinette's 20v, though both are well above the 18v shown on Fender's 5C3 Deluxe layout (https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5c3.pdf).
So what? The 5C3 Deluxe would distort when receiving an 18v peak signal, but your amp will need a 21v peak signal to distort. That could be part of the "headroom" you're experiencing. Except we can't just reduce the cathode voltage, because tube plate current would go up, and your bias probe already indicates 364v x 0.040A = 14.56 watts (hot, but fine/typical for a tweed Deluxe). And FWIW, I also have some Eurotubes bias probes, and can confidently report they measure plate currently only (not cathode current) and plate-to-cathode voltage. Therefore, they give you the exact information you would prefer when performing multiple manual steps (it's also why you report "40mA" but your cathode voltage would seem to imply "42mA" per tube).
What to do? You could simply make the 4.7kΩ resistor between B+1 and B+2 a larger value. The 5C3 Deluxe used a 10kΩ resistor here, but your amp has higher voltage at B+1 (345v) than the 5C3 Deluxe did (308v). You might choose to get a few 5w resistors in 10kΩ, 15kΩ, etc. Then you could try them out & see which brings down the 6V6 dissipation and increases the power-section sensitivity in the way you prefer.
Never built one of these, so disclaimer up front, but isn't V1 meant to be a 12AY7?
This is true. Though the 12AX7 would be higher-gain, and we might expect folks to interpret this as "less headroom."
Also, Fender always used the same parts-values for their preamp circuit whether there was a 12AX7 or a 12AY7 in the socket. So while folks may be right to point out these types should probably have different parts-values to "optimize" their use, that's not what we see done in historical amps.
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I built one of these last year this time and the thing is waaay too loud at the point of any breakup. I resorted to overdrive pedals and that works great. I tried a Weber small attenuator and didn't like the spongy farty loose bottom end. With pedals it's awesome. Really good thump for heavy stuff. I don't play clean so this is a pedal only amp for me. Probably was a mistake to build in hindsight. I'm a Marshall guy.
Edit: I posted my voltages in bold beside yours. My PT is 290BX so it's slightly lower HT which makes sense looking at my numbers below. My OT is 1760E. After much research I took the suggestion of someone here to use the 290BX. There's a thread on it.
The voltages I (OP) have are:
B+1 387 375
B+2 345 324
B+3 270 238
V1 12AX7A
#1 147 129
#3 1.47 1.7
#6 174 131
#8 1.4 1.7
V2 12AX7A
#1 184 147
#3 1.39 1.3
#6 219 190
#7 15.5 17.8
#8 50 47
V3 & V4 6V6GT
#3 379 368
#4 343 368
#8 21 20.1
Here's a thread showing my plate dissipation.
https://el34world.com/Forum/index.php?topic=29644.msg326438#msg326438 (https://el34world.com/Forum/index.php?topic=29644.msg326438#msg326438)
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Hi again everyone!
I'm sorry I didn't post back sooner, but I did want to thank everyone who replied for their input and insight. I'm getting the impression that my build is doing what it's supposed to, and the voltages are not a concern. I'm thinking that just to play it safe, I'm going to cool the bias with a 270 ohm resistor instead of the 250. Other than that, I'm really happy with the build.
Thanks again for everything!
cc
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Hello again everyone!
If anyone is keeping score, here's an update on my optimized 5E3 build and bias:
Using the information provided by the venerable Uncle Doug, I measured and calculated the plate dissipation with 3 different cathode bias resistors. From what I understand, the plate dissipation for 6V6GT should be at 14 watts or under, correct?
250 ohm: Outer Tube 16.202, Inner Tube 15.046 (too high)
270 ohm: Outer Tube 15.30, Inner Tube 15.04 (still too high)
300 ohm: Outer Tube 14.82, Inner Tube 14.097 (Pretty close)
Should I keep going to a 330 ohm, or does anyone think I'm safe with a 300?
Thanks for all you time and advice.
cc
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that's as high as the dissipation will get, once you wail on a cathode biased (self biased) amp, dissipation tends to drop.
I would keep going only to see there is a point of diminished return between Plate voltage, cathode voltage and the effect on compression/clipping coming into play. scopes are real handy for that part, good ears will get you close enough.
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... here's an update on my optimized 5E3 build and bias:
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Should I keep going to a 330 ohm, or does anyone think I'm safe with a 300? ...
If you want faster breakup, I think you should keep the 250Ω cathode resistor and increase a power supply resistor:
What to do? You could simply make the 4.7kΩ resistor between B+1 and B+2 a larger value. The 5C3 Deluxe used a 10kΩ resistor here, but your amp has higher voltage at B+1 (345v) than the 5C3 Deluxe did (308v). You might choose to get a few 5w resistors in 10kΩ, 15kΩ, etc. Then you could try them out & see which brings down the 6V6 dissipation and increases the power-section sensitivity in the way you prefer.rts-values to "optimize" their use, that's not what we see done in historical amps.
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Thank you for the advice!
I'm getting the feeling that I'm going to be better off trying to reduce the B+ voltage as opposed to changing out cathode resistors.
I was checking out some stuff on Rob Robinette's site regarding lowering the voltage:
"It is also possible to drop your B+ voltage using a dropping resistor before your first filter capacitor. To take 20 volts off the 5E3's B+ you can use a 3 or 5 watt 220 ohm resistor (this applies to a standard 5E3). "
Should I check that out first before trying 10k, 20k and others?
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Using Rob's suggestion will lower the plate voltage as well as all other voltages in the amp. Since the amp's entire current will pass through this resistor you must use a fairly high wattage resistor, 10 watts typical.
Using HBP's suggestion does not change the plate voltage. It only affects the screen voltage (and other downstream node voltages). The screen voltage has a big influence on the amount of current passing through the tube. Lower screen voltage will lower the cathode current, thus dropping the plate dissipation. Usually, a 2 watt resistor is all that's required.
Either method will lower the tube power dissipation. I suggest trying both methods and use your ears and meter to see which you prefer. You may even prefer a combination of the two methods.
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Sluckey,
Thanks for the response. I think I like the HBP recommendation better, as I think Rob's will require me to fly a resistor off the board, which I'm not so hot on. I'll order a variety of resistors to try out.
Should I use wirewounds, or will metal oxide suffice?
cc
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I would use 3 watt metal oxide (just because I have a pretty good selection in bench stock). Hoffman sells a pretty good selection.
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Super, thanks for the advice!
cc
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Hi everyone!
I know it's been like forever since this thread was posted, but I wanted to finally provide an update for anyone still interested to see how this turned out. I put the 250 ohm cathode resistor back in and, finally got around to trying different 3 watt dropping resistors and settled on a 22k. My plate dissipation is now 12.51 on the inner tube and 13.27 on the outer. I think that's good for a 5e3 right? I'm going to play through it a bit today and see how it fares.
Thanks again for everything!
cc