biasing an AC30

[JC@]

Is there a prooved and safe point where to bias these amps?
I mean how many mA per tube without loosing the pseudo class A character of the amp without burning tubes in six months.
Thanks

[mytco]

Not too technical an answer but... I once worked on an AC30 for a guy who was concerned about tube life so I increased the value of the bias resistor to cool it down a bit.

The guy was instantly complaining that the amp lost some magic or as he put it "kraaang". I put it back to stock and he was most happy. FWIW.

[Liam]

The Vox schematic calls for 10 volts at the cathode resistor, which is a bias current of 0.2 A overall, or 50 mA per tube. I have just sold my AC30, but with JAN Phillips NOS tubes in it I gigged regularly for years on the same set of power tubes.

I always ran with the standard cathode resistor, in fact it might have even been a 47 Ohm. I was told the tubes wouldn't last, they didn't. Then I connected the speakers to the correct windings on the OT (Doh!) and had no further problems.

Liam

[JC@]

Originally Posted by Liam

The Vox schematic calls for 10 volts at the cathode resistor, which is a bias current of 0.2 A overall, or 50 mA per tube. I have just sold my AC30, but with JAN Phillips NOS tubes in it I gigged regularly for years on the same set of power tubes.

I always ran with the standard cathode resistor, in fact it might have even been a 47 Ohm. I was told the tubes wouldn't last, they didn't. Then I connected the speakers to the correct windings on the OT (Doh!) and had no further problems.

Liam

Liam, I had 344 volts on the plates with the stock 47 ohms resistor.
So the power transformer in this amp I worked in had a few more windings I guess.
I have changed the cathode resistor to 60 ohms and now is more or less where it should. Hot, but not that much.
This amp has two 8 ohm speakers, that I connected in series to the 8 oHm tap. The other way would be 4 ohms but the transformer only has taps for 8 and 16 ohms. Do you think this is ok?

[Liam]

I'm never all that sure about mis-matching speaker taps on OTs. I'm kind of hoping that you meant you connected two 8 ohm speakers in series to the 16 ohm tap. As far as I can recall that was standard AC30 wiring and worked just fine for me. If you mis-match the speaker impedance you will be giving the OT and the valves a load of work they don't need. How much bias current is it running at the moment? (Old AC30s take a LONG time to fully warm up. I always check cathode voltage after 15 minutes. They are a 25 watt amp for the first 10 minutes after turning on IME.)

[JC@]

Yes that's right, two speakers in series to the 16 ohm tap. Blue wire.
I'll double check what you mention about measuring current after 15 minutes or so.
Many thanks for your advise.

[TD_Madden]

sorry to hijack....bit it's related! (somewhat)

I've got a '66 AC50 coming from the West Coast and I'm interested in a similar pursuit...best bias range. I've even purchased some NOS Tesla (NOT JJ) EL-34s and sure want to take advantage of the under-rated TONE of this baby.

Recommended bias, given this scenario? Problem areas to look for? I was thinking about an Avatar 2x12 cab with Celestion Golds....

Thanks in advance.

[Liam]

Also related but not the same - had a fixed bias Hiwatt Custom 100 until recently, and with modern tubes in it (Sovtek EL34) it would "run away" really badly. Lost a couple of power tubes that way. I put in a voltage doubler and a pot for bias voltage adjustment, and biased it at 36 mA per tube IIRC, and it behaved fine after that. It was the hottest I've ever biased EL34s, but it sounded quite good. I sold it about a month back, and the new owner is going back to the original bias circuit. He's got some NOS Mullards, which I think is the only way you'd get away with it.

If you're interested I can get him to check what the actual bias current was on that amp. With adjustable bias it's quite interesting to bias as cold as you can, and then bring up the current until it sounds sweet. You'd be amazed how low that can be on some amps.

[JC@]

Originally Posted by TD_Madden

I've got a '66 AC50 coming from the West Coast and I'm interested in a similar pursuit...best bias range. I've even purchased some NOS Tesla (NOT JJ) EL-34s and sure want to take advantage of the under-rated TONE of this baby.

I don't have much experience with this amp, although according to the schem it seams a bit on the outer safe edge for modern production tubes.
You will see 470 vots on the plates and 450 on the 2nd grids.
According with vox you should bias to measure 22 volts dc across each 47 ohm cathode resistor.

*Real* nos Teslas sound nice but you shouldn't bang them while they are hot. Nos Teslas didn't have a rounded glass top like JJ's and such. *Real* nos Svetlanas are more sturdy and stand high voltages better.

[Alex R]

I see a lot of older AC30s and imo not to mention ime they are running too hot at 50ma per valve for modern production valves to cope easily - even JJs. 65 ohms tames them a little without too much de-kranging. Change the bypass cap too if it's old; even at 11 volts I once found one conducting DC and heating things up.

While I'm at it - if something isn't sounding quite right, and you see any little black cylindrical Hunts caps in there, suspect them first. I rarely have checked one good, and now change them on sight.

[JC@]

Originally Posted by Alex R

I see a lot of older AC30s and imo not to mention ime they are running too hot at 50ma per valve for modern production valves to cope easily - even JJs. 65 ohms tames them a little without too much de-kranging. Change the bypass cap too if it's old; even at 11 volts I once found one conducting DC and heating things up.

While I'm at it - if something isn't sounding quite right, and you see any little black cylindrical Hunts caps in there, suspect them first. I rarely have checked one good, and now change them on sight.

Yes I did put a 60 ohm resistor and it was still a bit hot but as I had 344 volts on the plates I didn't loose much kraang if any. This amp's power transformer was definetely hotter than others.
All cathode bypass caps changed including of course the 220/25 one.

[TD_Madden]

think that was 2.2vdc across each 47 ohm reistor on the AC50 :-)

Received it today and it runs HOT HOT HOT, even with the 2.2v bias....and with "real" Tesla EL34s.

[Liam]

But more importantly, what does it sound like?

[TD_Madden]

LOUD and CLEAN

[TD_Madden]

have to track down some excess hum in the "brilliant" channel as well......

[TD_Madden]

Thinking about an Avatar 2x12 8 ohm cab with a Eminence Redfang and Wizard installed....

[slideman82]

Originally Posted by JC@

I don't have much experience with this amp, although according to the schem it seams a bit on the outer safe edge for modern production tubes.
You will see 470 vots on the plates and 450 on the 2nd grids.
According with vox you should bias to measure 22 volts dc across each 47 ohm cathode resistor.

*Real* nos Teslas sound nice but you shouldn't bang them while they are hot. Nos Teslas didn't have a rounded glass top like JJ's and such. *Real* nos Svetlanas are more sturdy and stand high voltages better.

Be careful!!!!! The maths shows around 47 mA of quiescent current on EL34 cathode resistors (47ohm)... that's around 2.2V, not 22!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! The point is not quite visible on the 1965 schematic! But I think it does in the 1976 schemo. for better working, please take a look at the EL34 manufacturer's datasheet, where it should say the quiescent current for that voltage.

[Liam]

Originally Posted by slideman82

Be careful!!!!! The maths shows around 47 mA of quiescent current on EL34 cathode resistors (47ohm)... that's around 2.2V, not 22!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

It's OK, that one already got picked up by the man with the amp.

Originally Posted by TD_Madden

have to track down some excess hum in the "brilliant" channel as well......

For me that's one of the best bits of getting a "new" old amp, just being able to dig in and have a play without worrying too much about how much time you spend on it. I especially like it when people say "it doesn't sound very old for what it is".

[JC@]

Originally Posted by TD_Madden

think that was 2.2vdc across each 47 ohm reistor on the AC50 :-)

Received it today and it runs HOT HOT HOT, even with the 2.2v bias....and with "real" Tesla EL34s.

Sorry

[Liam]

While I'm waiting for my new toy to warm up so I can recheck the bias...

With 470 V on the plates 47 mA bias current you're dissipating about 22 W a tube, which isn't terrible for a quasi class A amp (I only hope RG doesn't read these threads because I'll get castigated for that one!) I generally use the OT primary resistance to measure bias current, so that I'm only looking at the current through the plate. That way I can work out roughly what plate dissipation is.

For a coolish Marshall EL34 bias I run 15 watts, haven't yet got a clue what
6L6 tubes are going to want...better go and see if those plates are glowing yet!

Liam

[TD_Madden]

By the way, a word of warning about the NOS Tesla EL34s....apparently the center key isn't very well-defined, making it pretty easy to insert the tube incorrectly. Mike Kropotkin includes a warning notice with these tubes....putting one of these tubes in wrong can QUICKLY redplate it!

Ask me how I know this and how fast I can unplug an amp!!! No harm done this time, but a real scare, especially when the tubes are 80 bucks each!

I did bias to 2.2vdc across the 47r, but I'm used to seeing bias portrayed in a more standard way...like in ma. How can I connect in order to see that type of display on my Fluke?

In that regard, even set at 2.2v, the smell of "old amp" is really obvious; hope I'm not on the verge of an OT meltdown! Another good reason to see the milliamps!

[slideman82]

Originally Posted by TD_Madden

By the way, a word of warning about the NOS Tesla EL34s....apparently the center key isn't very well-defined, making it pretty easy to insert the tube incorrectly. Mike Kropotkin includes a warning notice with these tubes....putting one of these tubes in wrong can QUICKLY redplate it!

Ask me how I know this and how fast I can unplug an amp!!! No harm done this time, but a real scare, especially when the tubes are 80 bucks each!

I did bias to 2.2vdc across the 47r, but I'm used to seeing bias portrayed in a more standard way...like in ma. How can I connect in order to see that type of display on my Fluke?

In that regard, even set at 2.2v, the smell of "old amp" is really obvious; hope I'm not on the verge of an OT meltdown! Another good reason to see the milliamps!

If you think it's getting really hot, bias it at a minor voltage! Also, listen to the sound, if it is sounding ok for that bias voltage

[Liam]

I did bias to 2.2vdc across the 47r, but I'm used to seeing bias portrayed in a more standard way...like in ma. How can I connect in order to see that type of display on my Fluke?

I love cathode resistors for that, but you have to do a bit of math. Just use current = volts / ohms. 2.2 V / 47 ohms = 0.047 A , which is 47 mA.

If you want to read directly in mA put a 1 ohm resistor in series with the 47 ohm cathode resistor. Then if you measure dc volts across the 1 ohm with your Fluke you get the current. I.E. 0.033 V is 0.033 A, or 33 mA.

HTH

Liam

[slideman82]

Originally Posted by TD_Madden

think that was 2.2vdc across each 47 ohm reistor on the AC50 :-)

Received it today and it runs HOT HOT HOT, even with the 2.2v bias....and with "real" Tesla EL34s.

Another tip! I don't know why AC50's factory schemo affirms 47 mA of quiescent current... EL34 has to be set around 39, I've read it in many places... If it gets hot, do the maths and re bias it! Check on http://www.aikenamps.com/Why70percent.html

I'm interested on this, because I want to build myself an AC50!

[TD_Madden]

Mike Kropotkin sent me a note stressing that the NOS Teslas should be biased for 15-16 watts MAXIMUM each.....

At 47mA, they were putting out almost 24....and their maximum is 25.....pretty dangerous to the plates!

[slideman82]

Originally Posted by TD_Madden

Mike Kropotkin sent me a note stressing that the NOS Teslas should be biased for 15-16 watts MAXIMUM each.....

At 47mA, they were putting out almost 24....and their maximum is 25.....pretty dangerous to the plates!

Ok! Pretty good info! I didn't know about that!

[Liam]

EL34 has to be set around 39, I've read it in many places...

Whoa, steady there Slideman, plate dissipation depends on the plate voltage as well as the current. Power = current x voltage. 39 mA would kill either of my Marshalls' tubes in no time as they have quite high plate voltage.

I've been using 50 - 70 % of max plate dissipation for idle since I read that excellent Aiken article years back, but some amps were designed to be run quite hot. You really do need to use your ears as well as a meter to find what works.

[TD_Madden]

just rebiased and the Teslas are idling at around 30mA each at 499-500v...

that's about right for 15 watts each. Perfect.

Now to find and remove the residual hum.....

[Liam]

Just been doing this on my new toy. First place to look is generally the first preamp tube IME. On my Marshall BBRI today it was heater wires too close to the plate wire on pin 1. Moved them so they crossed at right angles and were as far apart as possible, and it's pretty quiet now.

(You know the sort of day when the "no-name" NOS ECC83s you had been saving for a rainy day turn out to be 60s Mullards Just had one of those)

[TD_Madden]

yes, this thing had a couple "Silvertone" 12AX7s...made in Holland.

[guitaraid]

Originally Posted by TD_Madden

Mike Kropotkin sent me a note stressing that the NOS Teslas should be biased for 15-16 watts MAXIMUM each.....

At 47mA, they were putting out almost 24....and their maximum is 25.....pretty dangerous to the plates!

Hi guys,
Just found this forum. I'm new here, so Hello to all of you.

47mA through cathode resistor means one valves takes roughly 11-12mA.
So 12mA and 470V gives less than 6watts for each valve. Am I right?
It's OK for A class, I guess. No worries you don't bias them at their maximum power.


Regards,
Greg
www.guitaraid.co.uk

[slideman82]

Originally Posted by guitaraid

Hi guys,
Just found this forum. I'm new here, so Hello to all of you.

47mA through cathode resistor means one valves takes roughly 11-12mA.
So 12mA and 470V gives less than 6watts for each valve. Am I right?
It's OK for A class, I guess. No worries you don't bias them at their maximum power.


Regards,
Greg
www.guitaraid.co.uk

Hi Greg! I've just read this and it seemed interesting to me, I mean, your analisis. But, the fact is, the AC50 maybe was biased it just to work near class A operation, and that's why the primary impedance of the OT is 3,5K not 5,6K or similar, like others. Take a look at the resistor values, they are not quite common.

So, the 47 mA through EL34 could be a correct value, but maybe not very safe. Some guy told me that EL34 in class A get very hot.

[guitaraid]

Originally Posted by slideman82

Hi Greg! I've just read this and it seemed interesting to me, I mean, your analisis. But, the fact is, the AC50 maybe was biased it just to work near class A operation, and that's why the primary impedance of the OT is 3,5K not 5,6K or similar, like others. Take a look at the resistor values, they are not quite common.

So, the 47 mA through EL34 could be a correct value, but maybe not very safe. Some guy told me that EL34 in class A get very hot.

Well I was referring to AC30, not AC50, where there is only one 50 ohms resistor for all valves. Not one per valve as it is in AC50.
And you are right - there is much higher idle current in class A, as valves are always 'on'.

Cheers

[slideman82]

Originally Posted by TD_Madden

LOUD and CLEAN

Yeeeeeah! that's what I'm talking about! In MY theory, amp must sound clean, and you make distortioned sounds from the floor ----> stompboxes!
unless you want output tubes distortion. If you want more dirt in your AC50, just remove the 220k resistor from the whipper of the treble pot (ok, remove it, and then solder that cable to the whipper). That resistor atenuates the signal from the preamp, so the amp sounds clean. The use of a 12AU7 as 1st stage contributes to that, a 12AX7 will make it dirtier, try it, also 12AT7, etc.

[slideman82]

Originally Posted by TD_Madden

think that was 2.2vdc across each 47 ohm reistor on the AC50 :-)

Received it today and it runs HOT HOT HOT, even with the 2.2v bias....and with "real" Tesla EL34s.

My bad, dog! I took a look to the 1976 AC50 schematic, and it says 1.8V over the 47 ohms resistors, so, that's about 39 mA, about 18W of dissipation. I think that's the right vlaue, not 2.2 V. Regards