Ideal EL84 tubes for a balanced tone and smooth overdrive in a JMI era AC30 TB with 50r cathode resistor?

[VintageCharlie]

Hi!

I have a 64 VOX AC30 TB with Albion transformers which has become my favorite amp of all time over the last years. But i made the discovery (for the first time in my life so clearly) that the output tubes make or break the sound for me. This was a suprize to me as i thought that people usually obsess more about preamp tubes, v1 in particular.

I bought the amp from the previous owner with mysterious old el84 tubes (supposedly Japan made) of which i do not know the hours they have been used nor what specs they are or how well matched they are, but they sound spectacular in that amp.

I bought a fresh set of JJ EL84's from Tube Town which they grade as #7 and that are supposedly intended for AC30 amps among others (https://www.tube-town.net/ttstore/jj-el ... anguage=en). While the old Japanese El84's are perfectly balanced in the amp and sound full and smooth, the JJ's seem to overemphasize the upper midrange and loose bass/low midrange. They have more output, overdrive more and produce more but unpleasant (to my ear) harmonics.

Here are short recordings of the different tubes in same amp with untouched settings and with identical recording setup. The input was from a looper pedal, so also that part is identical. The only variable are the output tubes. I was and still am astonished by the difference, as i have not heard such a dramatic difference in any tube comparison online. I mean, it is not a difference that you need to directly A/B - you hear it, if you know how the amp sounds with the other tubes, even if you would not have played the amp for a month. It is even more apparent live, where the overdrive with the JJ tubes was scratchy and everything but smooth. The difference is of course most apparent, when comparing the clips A/B in a DAW, where you can switch instantly between both tracks.

https://soundcloud.com/vintagecharlie/s ... 64-ac30-tb

Here is an older demo of the same ap with the MIJ EL84 tubes. The youtube video lost a lot of the punch and loudness of the original audio, but it should still communicate the tone this amp gets with the old Japanese tubes:

https://www.youtube.com/watch?v=kP4sm_8iRdU

What would be the best approach, if i want to get the tone of the old tubes, with less output, more balanced and smooth overdrive tone (less upper midrange harmoncis)? I am looking for options in current production tubes that are readily available. What are the parameters i should be looking for? Is it also a question of brands or rather specific characteristics of the quad that i use? Does the current draw specified by the seller according to which they are graded tell anything useful, or is it an arbitraty number for the specific use in an AC30? Should i consider changing the cathode resistor to 80r - would this make the JJ tubes sound closer to the old EL84 (less output, less overdrive, less harmonics, more balance and smoothness)?

Any input is much appreciated - thanks!

[Stevem]

The first thing I would try while the JJs are in the amp is to lift one end of the bypass cap that's across the output tubes cathode resistor.

[VintageCharlie]

The first thing I would try while the JJs are in the amp is to lift one end of the bypass cap that's across the output tubes cathode resistor.

I measured the following with the old tubes and will repeat that with the JJs soon:

Cathode 9.48v
Heaters 6v
Plate 316v
Screen 309v
Cathode bias resistor 8.80-9.15v (53r) (12.7-13.2w per tube depending on the voltage drop of the bias resistor.)
Screen resistors measure around 120r

Around 270v coming from the amps HT at 228v wall voltage and amp set to the 245v tap (to lower the voltages slightly)

I was thinking to try the following in that order:
- adjusting bias resistor to lower the plate dissipation which is probably too high for JJs
- Try to reduce plate/screen voltage via VVR

What is the idea of lifting the bypass cap? Btw. it has been replaced with a modern rifa cap by the previous owner.

[pdf64]

Please can you clarify the cathode bias arrangement?
Is your valve dissipation calculation in regard of anode only, or anode + screen grid?

[VintageCharlie]

Please can you clarify the cathode bias arrangement?
Is your valve dissipation calculation in regard of anode only, or anode + screen grid?

I only used plate/anode voltage. Please see attachment for the calculation.

I am not sure, if I understood your question regarding cathode bias arrangement. If it concerns the schematic - it is a stock AC30 Top Boost.
I measured the bias resistor at 53r and the voltage drop varies between 8.80-9.15v at idle.

[pdf64]

...
Cathode 9.48v
...
Cathode bias resistor 8.80-9.15v (53r) (12.7-13.2w per tube depending on the voltage drop of the bias resistor.)
...

Confused by the above, where does the 9.48V cathode voltage come from?
Are you are subtracting screen grid current from the cathode current?

[Stevem]

Several sonic factors could be at play here!

Here are some possible explanations in no particular order for what you are hearing and feeling .

New good... not even very strong testing tubes have a much faster response time to signal being applied to them then the old weak tubes you had.

This factor as I have made note of myself can influence perceived smoothness of the sound of a amp, especially when the amp is being driven hard.

JJ tubs are known to have a nice wide extended frequency range which in terms of harsh high end reproduction of clipped signal being put into them by the PI section may be part of your issue with them.

New tubes have more gain, and to your ears you may not want the output stage to have that level of influence on your sound and the amps play feel.

Different chemistry construction of a tube plate material can make them when driven hard produce a different mix of overdrive harmonics ( 3rds, 5ths, 7ths and such) then what you like out of the old tubes you had in the amp.

So there's some things for you to ponder which I hope may help!

[pdf64]

...
New good... not even very strong testing tubes have a much faster response time to signal being applied to them then the old weak tubes you had.
...
Different chemistry construction of a tube plate material can make them when driven hard produce a different mix of overdrive harmonics ( 3rds, 5ths, 7ths and such) then what you like out of the old tubes you had in the amp...

The above points seem a bit 'out there' - how have you come to those conclusions?

[TUBEDUDE]

Worn tubes lose hi end and get weaker. The new tubes will have greater dynamics and sound brighter. This can make the attack crisper and feel "faster".

[VintageCharlie]

...
Cathode 9.48v
...
Cathode bias resistor 8.80-9.15v (53r) (12.7-13.2w per tube depending on the voltage drop of the bias resistor.)
...

Confused by the above, where does the 9.48V cathode voltage come from?
Are you are subtracting screen grid current from the cathode current?

The 9.48v was a measurement of pin 3.
For the calculation i used the voltage drop on the cathode bias resistor.
No, i was not subtracting screen grid current from the cathode current. I used the calculation as shown in the excel screenshot above. Does it need adjustments?

[VintageCharlie]

...
New good... not even very strong testing tubes have a much faster response time to signal being applied to them then the old weak tubes you had.
...
Different chemistry construction of a tube plate material can make them when driven hard produce a different mix of overdrive harmonics ( 3rds, 5ths, 7ths and such) then what you like out of the old tubes you had in the amp...

The above points seem a bit 'out there' - how have you come to those conclusions?

It is not an unlikely scneario - i really do not have any clue how many hours are on these tubes. I will order some matched quads soon to compare. Probably no other around it. There seems to be a tendency of people atributing harsher high mids to JJ tubes and claims that the various New Sensor / Xpo-Pul factory brands being smoother and darker with less of these aggressive harmonics. Will try a couple - Tung Sol and Mullard RI above all probably.

Meanwhile i measured the primary windings of the OT and switched the output tubes in order to see how closely they are matched. My tech calculated the results and said they are matched within 10%, which is OK even for todays standards - so there is no huge mismatch. A big number of hours on them might still be a factor. Attached is a chart of the measurements i took. My tech made the calculation part.

[martin manning]

Looks like there is one combination you missed (2413), but if I'm interpreting your data correctly it doesn't matter, the first arrangement (1234) has virtually identical current on each side.

[rooster]

https://www.thetubestore.com/preferred- ... emium-el84

I'm not sure what the OP decided on for EL84s but these are what I highly recommend. The price is crazy, but look carefully at the tube construction (note the 3 legged getter at the top of the tube) and you may find cheaper prices from European sellers. This tube in an AC30 is a beautiful thing. BTW, my AC30 is looking at 325VDC on the plates and my bias resistor is 60 ohms. With the plate voltage you describe I wouldn't even bat an eye at that 50 ohm resistor. 40 ohms might suit you better here, but why have you elected to reduce the voltage so much? I think this is erring on the side of trying to turn an AC30 into something less than an AC30. Too, you're changing the bias on the preamp tubes as you do this and this is getting away from the Dick Denny design. All the best, I hope you sort things.

[VintageCharlie]

Thanks for the hint! I've heard these are great. Well, i tried even original old Blackburn Mullards and i guess i just don't like the overdrive characteristic of this tube. Works great for lots of stuff, but the fizz and sand-like overtones drive me crazy. I am now experimenting with a KT66 output stage and an AC30 preamp.
EH EL84 sounded good to me - better balanced than JJ, which were a bit boomy and not in a good way for the most part. But i am sure that is all subjective. There was really noticeably loads more bottom with JJ's - good, if anyone is afte that effect.

[rooster]

Oh, I should have mentioned that I also run an Albion OT, but one made by MM 20 years ago, so not original. It's a very nice OT and takes a beating quite well. They say the real deal Vox trannies were potted not in lacquer but some sort of beeswax concoction that, once the amp got super hot the paper bobbin and the beeswax would light on fire. (I will assume there was a power tube short in there somewhere that arc'd the OT winding.) I think it's Gerald Weber who presents this Vox beeswax OT fire somewhere in print, look around. He is possibly a conman on some level, true, but even so it is a good story. And yep, I remember quite fondly the time I was playing a gig (when I was 14) and the 6/10 four 6L6 piggy back Silvertone burnt up one of it's OTs. No flame, just a very solid stream of thick smelly black smoke rising to the gym ceiling. It happens.

But a good AC30 roars like a lion when it's right so maybe a little fire and flame should come with it, eh? Good luck with you amp.