Tung Sol 6v6 Reissue- underspec'd?

[EETStudent]

Ive been building a cathode biased 6V6 bassman build, and Im having concerns over a new set of Tung Sol 6V6s. The B+ voltages are in the 5E3 ballpark, about 372V on both plates. The screens are sitting just below at around 371V.

I fully expected a 250 ohm cathode resistor to be sufficient, but I experienced some redplating almost immediately. So I tried a 300 ohm, and still got some redplating. That was the largest value I had. I find it hard to believe at these modest voltages that I should be getting redplating. Anybody else have this experience with Tung Sols? Is there something I can look into?

[Steve Conner]

Yes, measure the plate current and calculate the plate dissipation.

[pdf64]

'The B+ voltages are in the 5E3 ballpark, about 372V on both plates. The screens are sitting just below at around 371V'
What schematic are you using?
Reason being that 5E3 screens are fed via a 4k7 dropper, so are generally a good bit lower than the plate; as your screens so high, what arrangement have you used?
Screen voltage has a significant affect on plate current.
Pete

[EETStudent]

'The B+ voltages are in the 5E3 ballpark, about 372V on both plates. The screens are sitting just below at around 371V'
What schematic are you using?
Reason being that 5E3 screens are fed via a 4k7 dropper, so are generally a good bit lower than the plate; as your screens so high, what arrangement have you used?
Screen voltage has a significant affect on plate current.
Pete

That was one of my concerns. The power supply design is not 5E3, just the voltages. The power supply arrangment looks like an AB763( Deluxe Reverb values to be exact). Screens are fed from a choke between the first and second resevoir caps. My understanding is that running the screens closer to the plates will yield the maximum power output, and its recommended for improved tone. I cant say Ive experimented enough to form my opinion on that. I did go ahead and order a pair of NOS Sylvania 6V6GT. I figured they were a step up tonally and I could compare their biasing to the Tung Sols

[jrfrond]

You need to insert a dropping resistor in series with each screen to limit screen current. The Pi-filter arrangement with the choke runs the screens too close to the plates. The AB763 uses 470 ohm dropping resistors in series with each screen. The 5E3 used a 5K series resistor in the screen grid power supply, which translates to 2.5K per tube. With new issue tubes, I always up the screen grid resistors to 1K/5W from the usual 470 ohm used in BF/SF series, and still 'til today. If you were running NOS 6V6GTA, you stand a better chance of not having anything go nuclear on you. If not, you have to think about the state of new issue power tubes, many of which develop premature screen grid failure.

[pdf64]

Yes, I was thinking of this too.
'The 5E3 used a 5K series resistor in the screen grid power supply, which translates to 2.5K per tube'
My thinking is that a shared 5k is equivalent to individual 10k (screen grid resistors)?
Pete

[Bruce / Mission Amps]

Also, think of it this way.

Your choke is probably somewhere around 90 ohms to 100 ohms or so and the 5E3 uses a 4700 to 5100 ohm resistor.

At idle, the 4700 to 5100 ohm screen node resistor in the 5E3 is supplying current to both screens in the 6V6s plus all four triodes of the preamp stages.

If you allow, say, a total of 3ma or 4ma of idle current for the two screens of the 6V6s, (they might not actually draw more then 1 ma each at idle)... plus 1ma of idle current per 12A?7 triode, that is a total of as much as, 7ma to 8ma of current through your 90-100 ohm choke, rather then a 4700 ohm to 5100 ohm resistor found in typical tweed 6V6 amps.

If you recall Ohm's Law, it would show you that the voltage drop through that choke resistance would only be a volt or two DC with the choke.... and about 30 vdc or more with the resistor!!

100 ohms X .007a = <1volt drop, but 4700 ohms X .007a = almost 33vdc drop.
That will have an impact on your power tube's plate current, at idle, when the plate voltage is around 370v and the screens are 337vdc.

If the 6V6's screen current is really low, the total drop through the screen node might actually even be less then the drop through the output transformer at high idle current... in that case the screen voltage will be higher then the plate voltage!

Some mildly bad news for you is that the new plate voltage on your amp is going up when you drop the screen voltage down and unload some of the power supply B+ node.

The best sounding classic 5E3 amps I've heard have fairly low screen voltages and plate voltages less the 350vdc.

So keep a 5w to 10w, 300ohm, 330 ohm to 360 ohm cathode biasing resistor handy.

[Amp Kat]

What is the voltage across the Cathode resistor ?

[Bruce / Mission Amps]

Also.... we are assuming that you don't have something wired incorrectly... such as forgetting to "ground" the two 220K Ohm, 6V6 grid load resistors.... not a totally uncommon omission from my novice kit builders... ha ha :>)

[EETStudent]

All the wiring is definitely good. 220k grid leak resistors are grounded.

I'll have to double check the cathode voltage. With the 250 ohm Rk, it was about 23V. Cant recall what I measured with the 300 ohm.

Not sure if its important to note, but Im not really going for 5E3 tones in the amp. More like a 15-18W 6V6 Bassman. Tight, solid attack and mild compression from the power supply. Any loss in tone with additional screen resistors?

[Amp Kat]

All the wiring is definitely good. 220k grid leak resistors are grounded.

I'll have to double check the cathode voltage. With the 250 ohm Rk, it was about 23V. Cant recall what I measured with the 300 ohm.

Not sure if its important to note, but Im not really going for 5E3 tones in the amp. More like a 15-18W 6V6 Bassman. Tight, solid attack and mild compression from the power supply. Any loss in tone with additional screen resistors?

No not that I ever hear but if it's a huge jump you might would.

Saying you did have 23 volts across the resistor yields 92 ma x 371 = 34 watts / 2= 17 watts so for a 12 watt tube that is really close and maybe the RCA's will handle it better but those voltages for the 250 ohmer were actually designed for much lower than what is given in the schematics. The voltages went up and they never changed the cathode resistor to compensate for it.

[pdf64]

If you only want mild compression then consider fixed rather than cathode bias.
Pete

[EETStudent]

If you only want mild compression then consider fixed rather than cathode bias.
Pete

Compared to the type of compression found in a 5e3, I want something milder. I have an '66 Ampeg Reverberocket that is cathode biased with a tighter power supply, and the compression character is simply stunning. Very clear and punchy. It also has no negative feedback. I love the dynamics that cathode biasing and no negative feedback presents, and my goal is to produce an efficient variety of it.

[Amp Kat]

Actually the supply with the choke is tighter than the Ampeg and the only difference in the two are the 1st filter cap arrangement where the AB763 uses 32uf of filtering and the Ampeg 20uf and no choke. Two totally different preamp topologies as the Ampeg also uses 6SL7's and 6SN7's in the preamps and of course a ton of other differences but a beautiful tone monster especially with all NOS tubes is gold in that amp.

One thing you won't get with Cathode bias is efficiency and you may have to tweak it a bunch using current production materials to get it close at a price tag that comes with it but if thats not an option you just do what you can with what you have to work with. Bruce touched on something you may want to pay attention to and that's plate voltage so I would try dropping it a tad if you can maybe back biasing and see if you can get it to tighten up or reduce a little headroom with maybe a 12AT in the PI. You'll just have to experiment with it for awhile until you get what you want.

[Bruce / Mission Amps]

No not that I ever hear but if it's a huge jump you might would.

Saying you did have 23 volts across the resistor yields 92 ma x 371 = 34 watts / 2= 17 watts so for a 12 watt tube that is really close and maybe the RCA's will handle it better but those voltages for the 250 ohmer were actually designed for much lower than what is given in the schematics. The voltages went up and they never changed the cathode resistor to compensate for it.

Well, 372vdc-23vdc=349vdc.... looks just right to me.
.092a x 349vdc = 32 watts/2 tubes... 16 watts.... pretty high but not horribly and probably not deadly red plating territory for most tubes.
I still think a big part of this is your high screen voltage in a cathode biased amp using stock numbers where screen voltages were lower originally.
Also, I wouldn't be surprised to hear you have a few 6V6s that are soft and draw higher idle current then many others on top of that.