DIY 5F2A: Distortion, not loud. Where do I look?

Lowerleftcoast

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I had these parts from years ago and decided to try to put together a 5F2A.
Your measurements on the 6V6GT
PIN 3: 434 VDC
PIN 4: 372 VDC
PIN 8: 23.8 VDC
Plate to cathode voltage ~ 434 - 23.8 = 410.2
You have a 500R 6V6 cathode resistor. (We don't know the measured value.)

Using Rob Robinette's bias calculator for a 14W 6V6, the plate dissipation is approximately 132.1% of MPD.
(I like using Rob's calc because it is easy and it subtracts an approximate 5.5% for screen current.)
132.1 %MPD is very high. The tube may be able to take it. Maybe not. Sometimes tubes will do odd things when pushed past limits. I wonder if the harmonic sounds might be because of this.

I am curious about the filter capacitors. You say the parts came from parts you have had on hand from years ago. Capacitors have a shelf life. The harmonic sound is consistent with bad filter caps or poor connection to the caps. My spidey-sense has a concern with the two black/gray reservoir capacitors.
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6V6 can run with ~420VDC plate to cathode. 6V6 endure this on many amplifiers by design, (Princeton Reverb). So, imo, it is safe to keep the PT. The high voltage will make the 5F2A sound/act different than the low voltage tweed line it came from, but it will work and you may like it. We can't tell you if it is worth changing to a lower voltage PT.

My plan of attack would be to reduce the 6V6 current. A vintage rectifier tube might reduce the B+ enough to accomplish this. Changing the resistance of the 500R 6V6 cathode cap will accomplish this. This will increase the B+ a little but I doubt the 6V6 plate to cathode voltage will exceed 420. (Hopefully you have resistors on hand to change the resistance to around 600R.) You could also try a different 6V6. Chances are a JJ 6V6 would have a happy life in this amp.

If the harmonizing effect continues, focus in on the filter caps.

Once the amp is working correctly, you can decide if the high voltage 5F2A version is to your liking. If it is, then address the high heater voltages with resistors/diodes etc.

No one has mentioned a *bucking transformer* yet. This will reduce all of the voltages in the amp. It could solve all the high voltage issues at once.
 
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drew1d

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Funny enough, I have 4 of those lying around. I also have a 5AR4C. So will try..>

And I will work on the Cathode resistor per lowerleftcoast suggestions
I'm not an expert. I would say only make one change at a time. But I do know the 6l6 has a wide range of power handling capability.
 

King Fan

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Wow, what a journey -- you're doing a good job just to hang on, @heybluez74 . Sounds like you've fixed several things already. If it's me, about now I might be looking to make a list of further changes needed or wanted, and attack them in order of either 1) ease or 2) importance. :) Everyone here is a genius, of course, but not all our geniuses have synchronized their watches -- so I'd look for some consistency between experts, some citation of their source or reasoning, some specificity of instructions, that kind of thing.

Agree one change at a time. But you may have to make several, in series, and you'll save money on shipping if you can order things at least a few at a time. I'm especially interested in the advice from @Lowerleftcoast in post 41 -- he's always steered me straight.

My casual list so far (I may have overlooked all sorts of stuff):

1. NOS 5Y3
2. JJ 6V6 if you don't have one in there already -- they're way tolerant of high voltages
3. Try increasing cathode bias resistor to ~600Ω -- but measure all voltages again afterward
4. Practice calculating % max dissipation bias -- on Rob's calculator mentioned above the 'cathode resistor voltage drop' method is super easy.
5. I'd order some new filter caps (may improve sound, and they'll last longer, anyway)
6. Research heater-chain resistors, as mentioned by our smart friend @printer2 . High heater voltages are a great way to shorten the life of your tubes. Merlin's section on reducing heater voltage is good:

http://www.valvewizard.co.uk/heater.html

In the meantime, if you can't get bias and plate voltage to behave, or if you want a vintage sound, you may want lower voltages. While doing all the above, you can research a simple zener diode chain on the PT center tap -- Rob has a good discussion in his section about how to reduce B+ on his 5E3 mods page:

https://robrobinette.com/5e3_Modifications.htm
 

bwacke

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A suggestion from a non-genius:

Check the wiring on the 2nd input jack. It appears the ground is floating and the 1Meg grid leak resistor isn't really wired right. Drew1d was on to something ...or my eyes may be bad. The color code on one of the 68K resistors doesn't look right either ...or my eyes are bad. :)
 

printer2

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I didn't realize the OT could handle this! It can!

Since you have 6L6 waiting in the wings, I would try this before my other advice.
Do you think the PT can handle the increased heater current? 😅

Select the right tap for the 6L6, try 4k and check the cathode resistor bias. Adjust to taste, check in with your voltage and current.
 

heybluez74

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Do you think the PT can handle the increased heater current? 😅

Select the right tap for the 6L6, try 4k and check the cathode resistor bias. Adjust to taste, check in with your voltage and current.
what do you mean by:

Select the right tap for the 6L6?

and

try 4k? (assume this means the speaker output?)
 

printer2

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what do you mean by:

Select the right tap for the 6L6?

and

try 4k? (assume this means the speaker output?)
My brain or finger playing tricks on me, 5k.

125sesch.gif
 

peteb

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Are there concerns about an impedance mismatch?

the 6L6 and the 6V6 want to see different loads.
 

drew1d

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Are there concerns about an impedance mismatch?

the 6L6 and the 6V6 want to see different loads.
Well, a good OT should be able to hand a 100% mismatch without to much issue. If you wanted it to be closer, when you move from 6V6 on 8ohm speaker load, you could 6L6 to a 4ohm load like the Hammond chart above suggests. Or if your OT has multiple taps, you can just use the next speaker impedance down. As for filament load, it's .6 amps to .9 Yes it's more. I don't think too much...

If you don't want to try that route, I'm not offended. KingFan summed up a lot of things to try.

My suggestion wasn't really to fix anything, but to deal with the higher than normal voltage.
 

drew1d

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My brain or finger playing tricks on me, 5k.

125sesch.gif
I read Dave Hunter's "The Guitar Amp HandBook" and at the end of it, he has plans for a "Two Stroke Amp" (that amp borrows heavy from a 5F2a) that uses a Hammond 125ESE. That is the same chart that I found online for the OT. Basically, for an 8ohm load, [when using a 6v6, he suggest the yellow], and for [6L6 he suggest the white.]

This is probably not 100% accurate, but I'm not sure there is 100% accurate. Everything seems, at least to me, to be an approximation.
 

printer2

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I read Dave Hunter's "The Guitar Amp HandBook" and at the end of it, he has plans for a "Two Stroke Amp" (that amp borrows heavy from a 5F2a) that uses a Hammond 125ESE. That is the same chart that I found online for the OT. Basically, for an 8ohm load, [when using a 6v6, he suggest the yellow], and for [6L6 he suggest the white.]

This is probably not 100% accurate, but I'm not sure there is 100% accurate. Everything seems, at least to me, to be an approximation.
There is no proper impedance for a certain tube as what voltage they run at is important in selecting the impedance. The higher the voltage the higher the impedance. Always good to play with numbers,


420V 250V Screen 70mA 5k, does not look bad although 250V for the screen would need more than a dropping resistor.

330V 300V 90mA 2.5k seems reasonable.
 
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peteb

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Tube Amp Dr 5y3GT / 6087 TAD

I was going to post and I will post that that looks like a solid NOS tube that i would use, if Had it. It just has a little more power on tap than expected. Not a bad thing.


Looking closer, the description says


“The rectifier tube for smaller vintage amps like the 5F1 Fender Tweed Champ or the Tweed Deluxe 5E3 if more headroom is required.
Higher output voltage than the 5Y3WGTB (RT513).”


EDIT: this is identified as a TAD tube, and looks not to be NOS.
 
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peteb

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Tube Amp Doctor 5Y3WGTB Rectifier Tube​

Typical NOS sag.

5Y3WGTB TAD Selected can replace any 5Y3GT. It has that typical "sag" like a NOS 5Y3 due to its voltage drop when pushed, causing a somewhat reduced output power and lower volume. Replacement for 5Z2P




EDIT: this is identified as a TAD tube, and also looks not to be NOS, but acts like NOS.
 
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peteb

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This is the Tube Amp Doctor’s NOS 5Y3 offering:



Product information "5Y3GT/WGT/WGTB/GTA NOS USA"
5Y3GT/WGT/WGTB/GTA 6087 NOS USA
various brands, mostly made by Sylvania
 

peteb

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And this is the one to avoid at all costs even though the 15 Euro price tag sounds enticing.



I learned many years ago to avoid Sovtek rectifier tubes.
they never last.
 

peteb

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I see the Tube Amp Doctor has two high voltage and two low voltage 5Y3 rectifier tubes.


what is the difference between high and low voltage 5Y3 rectifier tubes?

the 6087 and the Russian 5Y3 are high voltage output.

the USA NOS and TAD’s select 5Y3WGTB with NOS sag drop more volts and are lower output.

Wikipedia says the original 5Y3, the original eight pin rectifier tube, has a directly heated cathode while the 6087 and the sovtek 5Y3 are indirectly heated cathode.




The 5Y3 is a medium-power directly heated full-wave rectifier vacuum tube introduced by RCA in 1935. It has found wide use in tube radios and early guitar amplifiers (of the Fender Champ type). It is virtually identical, electrically, to the older four-pin type 80 tube, but with an octal base. 5Y3G, 5Y3GT, and 5Y3WGTA are variants with essentially the same specification;[1] the 6087 is similar and a plug-in replacement, but with indirectly heated cathode


RCA and other manufacturers later introduced many similar 5V rectifier tubes of both higher and lower current and voltage rating, including the 5V3, 5W3, 5X3, 5Z3, 5U4, 5Z4, GZ32, and GZ34.

As for a suitable replacement, the 5Y4G has same specifications as the 5Y3G but with a different pin out.[2][3]

Sovtek in Russia manufacture a 5Y3GT with similar specifications,[4] but indirectly heated, and with higher maximum current at 140 instead of 120 mA.[dubiousdiscuss]





OK



same difference as the GZ34 is indirectly heated and has less voltage drop while the 5U4 is directly heated and has more voltage drop.


I think I remember reading that tubes may be sold or marked as 5Y3 but they really aren’t. They probably have less voltage drop.
 
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