6v6 amps - screen grid resistor values don’t match - does this matter?

[Twangisgood]

Hi to you all.
I’ve got a deluxe reverb (the 68 reissue). It’s got an arcing power tube (I did swap them round to check that it was just the one tube)

Having had a look inside them, everything was fine except that just happened to notice that the screen grid resistors were reading different values to each other. Is this what’s caused the mismatch between the power tubes when trying to bias the amp (both tubes give different readings)
I’m not an amp expert at all, but have been enjoying leading a bit about this over the past few years. I’m in the uk and it seems to be hard to find the correct 470ohm resistors to replace these with? I might be wasting my time, but all I can think is that the mismatch of these resistors caused my nice Tung Sol power tubes to now be useless!
Like I said, I could be wrong but I’m worried about putting in a new set of tubes and running the amp if these resistor values are critical to be matched? If so, I’ll have to buy some, match them and replace them before I can use the amp again.
But if a mission! But hopefully someone might have a much better idea?

Many thanks

 

[Jasonpatrick]

Meh, nah. Stick new tubes in and go. If it followed the tube then it’s the tube. If it’s the stock tubes from the factory, then ain’t matched worth a sht and honestly it doesn’t really matter as long as the are matched a little, like no more then 10mA ish Difference let’s say. The resistors , like how different? Like 470 and 500? 470 and 440? 200? 800?

 

[2L man]

Did you measusure resistors resistances and if you did what they are? I do not trust color bands on old amps because they can change but it they definitely are different most likely it is a build mistake. Severe difference might have caused unbalanced operation and resulting "flyback" voltage pulses wear out tube or cause its sudden destroy?

 

[Twangisgood]

Meh, nah. Stick new tubes in and go. If it followed the tube then it’s the tube. If it’s the stock tubes from the factory, then ain’t matched worth a sht and honestly it doesn’t really matter as long as the are matched a little, like no more then 10mA ish Difference let’s say. The resistors , like how different? Like 470 and 500? 470 and 440? 200? 800?

They weren’t the original stock tubes, I’d changed them about 5-6 months ago for new Tung sol’s. I’ll take a reading and let you know what they’re currently measuring.
Thanks

 

[Twangisgood]

Thanks for your replies.
The new power tubes have arrived (TAD 6v6gt ones), but before I put them in the amp and run it, I’ve measured the values of these screen grid resistors.
R-62 = 425 ohms
R-63 = 444.1 ohms
This means a 19.1 difference between them. I really though these would have been matched more closely, but like I said in the original post, I’m no expert but it might explain why when I biased my new Tung Sol’s 5 or 6 months ago, they wouldn’t match despite swapping them around etc. the mA’s readings were quite a bit off! Perhaps it’s been these grid stoppers (and they’re mismatched values) all along?
Wish I knew for sure!

 

[2L man]

I have two pairs of Tungsol 6V6GT reissues which have my favorite "6V6 sound". Unfortunately they are prone to "red plating" which high voltages and hotter bias tend to cause. I did drill holes to combo back plate so that tubes could cool but also that I was able to check them when playing.

I quit using TS 6V6 on amps which anode-cathode is more than 330V because they are expensive now and biasing them colder made them sound less interesting like other tubes. It looks like when voltage is 300V or less biasing TS 6V6 to 12W or but higher does not lead to redplating but bring good sound back. Naturally OT impedance and loadline and Screen voltage effect here and voltage alone when amp idle when bias is checked is just on "idle point".

 

[Twangisgood]

Thanks for your replies.
The new power tubes have arrived (TAD 6v6gt ones), but before I put them in the amp and run it, I’ve measured the values of these screen grid resistors.
R-62 = 425 ohms
R-63 = 444.1 ohms

I have two pairs of Tungsol 6V6GT reissues which have my favorite "6V6 sound". Unfortunately they are prone to "red plating" which high voltages and hotter bias tend to cause. I did drill holes to combo back plate so that tubes could cool but also that I was able to check them when playing.

I quit using TS 6V6 on amps which anode-cathode is more than 330V because they are expensive now and biasing them colder made them sound less interesting like other tubes. It looks like when voltage is 300V or less biasing TS 6V6 to 12W or but higher does not lead to redplating but bring good sound back. Naturally OT impedance and loadline and Screen voltage effect here and voltage alone when amp idle when bias is checked is just on "idle point".

Yes, they really are a great sounding 6v6. It’s a shame that they’re not really rated for the higher voltages in this amp. I did happen to look at the specs only after I’d installed them. I did the bias measurements when I put them in a few months ago. It was then that I noticed the mA’s were reading quite different values at that time, but I went ahead and left them in there. I’m thinking that might be the cause of the early tube death, but as you say it could just be the high voltage strain put on them. (I am just guessing of course!)

Looking at my mismatched values of the screen grid resistors above, would you think I should replace them before installing these new TAD summer of 69 specials I’ve just had delivered?

 

[Twangisgood]

Update, just checked the bias on each power tube (the new TAD ones I’ve just installed) and the meter is showing:
27.8 mA
25.3 mA
I realise that’s only a 2.5 mA difference between the two, but could these mismatched grid resistors be causing that small diffence in voltage to the power tubes? Hence me never being able to balance them properly and achieve a bit more of an accurate bias!

 

[2L man]

Bias is only one point on 6V6 loadline which typical push pull travel from 0 to 120mA and even higher when driven to high distortion. Good tube matching is done using wider operating range than just bias point.

Misbalanced push pull power tubes increase 2nd harmonic distortion which make quitar amp sound sweeter. Very good tube balance is needed for HiFi and when very fierce 3rd and further odd harmonic rich guitar sound is wanted.

There should be either a Choke or voltage dropper resistor between OT feed and Screen feed and filter capacitors for both so measure its value. Screen voltage has effect how efficient control grid is but on bias point Screen current is very low and in prantice both screens have same voltage.

You could cross change tubes and sometimes bias balance comes closer. Often OT primary halfs have some resistance difference which make tubes anode voltages bit different. Usually a tube which is stronger wear out faster and bias balance get closer. But sometimes they drift further each others.

 

[Paul G.]

Those resistors are fine. They are within 5% of each other. Voltage drop across these (assuming same current draw and 450V) will be <.2vDC between them.

 

[slider313]

Update, just checked the bias on each power tube (the new TAD ones I’ve just installed) and the meter is showing:
27.8 mA
25.3 mA
I realise that’s only a 2.5 mA difference between the two, but could these mismatched grid resistors be causing that small diffence in voltage to the power tubes? Hence me never being able to balance them properly and achieve a bit more of an accurate bias!

You didn't disclose your plate voltage. The Deluxe Reverb usually runs in the 430v-440v range.

If it's my amp, I would cool the current draw down to about 22mA per tube.

 

[JPKmusicman]

Those resistors are fine and that bit of difference in bias current is no big deal. And as mentioned, the OT is not perfectly wound, so each side will be slightly different. How did you actually measure the bias current? If you measure the warm OT resistance from CT to each side (amp off and discharged), and each warm plate to center tap voltage (amp on, be careful) you can calculate the bias current and plate dissipation to make sure you are not biased too hot. Just looking at bias current alone is not enough.

Here are the numbers from a Deluxe clone I just built. It's cathode biased but the same math applies. The amp sounds fantastic and the JJ 6V6S's have a nice orange glow to them.

Tubes: 2 x JJ 6V6S (matched set, max plate dissipation = 14 watts)
OT CTR tap = 373V
Plate 1 = 368V
Plate 2 = 368V
OT BRN lead to plate 1 = 151.0 ohms.
OT BLU lead to plate 2 = 164.1 ohms.

-Plate dissipation using OT resistance method (most accurate):
I plate 1 = (373-368)/151.0 = 5/151.0 = 33.1 ma. P = 368 x .0331 = 12.1 watts. 12.1/14 = 86%
I plate 2 = (373-368)/164.1 = 5/164.1 = 30.5 ma. P = 368 x .0305 = 11.2 watts. 11.2/14 = 80%

Set Bias

How to Bias a Tube Amp

robrobinette.com

 

[SnidelyWhiplash]

I like the TS RI 6v6s, but they aren't the most robust tubes. A DR has high plate voltage & the tubes in ? just can't hack it.

 

[schmee]

The resistors are fine, the bias is as close as you normally get.
Consider JJ 6V6's next time. Many amps that are 6V6 based run quite high in voltage.

 

[Twangisgood]

Thanks, as much as the JJ’s seem very robust, but I much prefer the other type of 6v6’s as I think the JJ’s don’t sound like a 6v6 should (did!).. Wish this wasn’t the only manufacturer who upped the rating of their tubes to handle the voyage these amps produce. Went for the TAD summer of 69’s, but we’ll see how long they last?!

 

[zook]

I have used Russian 6V6s with good results. I keep a few on the shelf. They are Military Surplus.

 

[Twangisgood]

Well, the plate voltage measured 437 on both output tubes. Oddly, since swapping over the new tubes and playing them in for a bit, they both are now pretty well matched. Only around a 1-3mA difference.
I've biased the tubes quite hot (26mA) which gives me the tone I like, so will see how it goes.

 

[slider313]

Well, the plate voltage measured 437 on both output tubes. Oddly, since swapping over the new tubes and playing them in for a bit, they both are now pretty well matched. Only around a 1-3mA difference.
I've biased the tubes quite hot (26mA) which gives me the tone I like, so will see how it goes.

26mA @ 437v is over 80% dissipation. It's excessive current that kills a tube more so than high voltage. I'm going to bet those tubes won't last very long. I've had bad luck with most power tubes made in China. Good luck.

 

[msalama]

Russian 6V6s

Yeah, they're good. Used to use them back in the day, when they were still available dirt cheap. And those milspec valves can withstand high plate voltages as well!

 

[Twangisgood]

26mA @ 437v is over 80% dissipation. It's excessive current that kills a tube more so than high voltage. I'm going to bet those tubes won't last very long. I've had bad luck with most power tubes made in China. Good luck.

I agree, it is hot at 81%.. I went through the Rob Robinette site and looked at the numbers there% It someone on that site made a quote saying 70% was ideal but a/b amps should be able to run at a max of 85%.. (Or words to that effect)
Is this wrong then? What would you suggest I bias it to instead?.. I love the clean sound all thick as it is. That’s the trouble!
I thought a lot of the original blackfaces were often running at 30mA or even more sometimes?