Sovtek Mig-60 bias - Tungsol EL34B

[alerich]

Someone check me on this. I have a Sovtek Mig-60 head. This one was modded by Scott Splawn of Splawn Amplification and really sounds very awesome. The thing that has me puzzled is the bias. These amps run at notoriously high plate voltages. This one is running about 640 VDC on the plates. I have Tung Sol EL34B power tubes in it that are rated at a max plate voltage of 800 VDC. These amps have 3.9 ohm resistors in the cathodes as opposed to the more common 1 ohm resistor. This amp was upgraded with Mercury iron but it they are the models MM has designed for this amp.

The schematic clearly states that you should set the bias to read 80mv across each of the 3R9 resistors. That comes to about 20ma and with a plate voltage of 640 VDC that will yield about 12.8 watts per tube at idle. I am not very familiar with EL34 type tubes. The max plate dissipation for the Tung Sol EL34B is listed at 25 watts. Based on the notes on the schematic this amp is designed to idle at about 25 watts. What the 60 in Mig-60 means is a mystery to me. People talk about how loud these amps are. It will get loud if I crank it but it is actually tame enough at normal master "settings" that I have not used my Hot Plate with it.

Is this a case that they are running the tubes a bit on the cold side because of the super high plate voltage? I thought dissipation was dissipation and was not related to plate voltage as long as the tube can withstand it. I know that the original Sovtek tubes that this amp shipped with were junk and the original OTs were not robust. Maybe that accounts for the meager suggested operating conditions.

[sluckey]

I agree. This amp is running very cool. That's probably because of the junk tubes.

[PRR]

Ampeg used a similar setup. Near 600V on EL34 plates BUT a much lower Screen voltage (from separate supply) and a high impedance load (~~6.6K) for 60 Watts out.

Idle bias has nearly no effect on FULL-power operation. You can idle cold and still get your 60 Watts when you play hard. If you play very soft, it might be sweeter with higher idle current; but you didn't buy a VT40 or a MIG to play soft, these are LOUD amps.

[alerich]

Yes, this amp uses a stacked power supply that supplies about 640VDC to the plates while running the screens around 350-375. Randall Aiken's biasing notes on his website also mention the importance of the OT impedance in uber high voltage applications. I always thought that a certain OT was the proper one to use with a pair of EL34 tubes (or whatever other tube you are using) and that was that. I learned something new today. I don't have the specs on this Mercury OT but it is a physically stout transformer for a 60 watt application. The PT is a brute, too.

[plexi50]

640VDC on the  plates is insane. I still get confused with the EL34M & B ratings. Man that has to be one LOUD amplifier

[PRR]

> I always thought that a certain OT was the proper one to use with a pair of EL34 tubes

Pages 4 and 5 of EL34 data (800KB PDF file). Also Attached.

Philips suggested values from 2,8K at 375V for 44W to 11K at 800V for 100W.

The 800V condition is unattractive because it is AT the 800V limit which is already silly-high.

Also all the 500V-up conditions require a lower G2 screen supply, an awkward complication.

The Basic Gitar Amp designer's eye goes to the 425V 3.4K 55 Watts condition. That's pretty good power, voltages are not distressing, G2 can be conveniently tapped from plate supply.  Of course 425V at full-power may be 450V-475V at idle. Bench-testing may show that this rig can stand 450V all night... not with military fighter reliability, but good enough for the stage.

The 2-tube "50W" Marshalls are off this condition. Voltage runs high and they are known to deliver 70 Watts not-too-bent.

For 100W, four EL34 makes more sense than a regulated 800V supply, an additional 400V supply, and a costly 11K OT. (Especially since Jim had a heap of 50W 3.4K OTs which could be simply paralleled for a 100W 4-bottle.) Also four bottles looks cooler and is perhaps more reliable than high-volting two bottles.

[alerich]

plexi50: Yeah, I had to do a double take when I was measuring voltages and immediately went to the schematic. There are no voltages on the schematic but a quick web search indicated that this pate voltage is nominal for these amps. I remember those old Music Man amps seemed to run fairly high B+ but I have never seen it on the bench myself. The amp can get loud but much of the lower sweep of the master knob is very usable. Not one of those amps that goes from quiet to wide open with a nudge. You have to get it past 12 o'clock before it really starts to cook. Well before that point on the dial my 100 watt amps are kicking ass and taking names.

PRR: Thank you so much for that info and explanation. This is really interesting information. It's another avenue on power that I had never seen or considered I guess because we don't see it that often in guitar amps. Good stuff.

[plexi50]

I had 525 PV on a Sovtek Mig 50 / 5-6 years ago. The Ampegs to me are the most dangreous and tricky amps to work on.  Have to pay extra attention when working on them. It's like when an SVT comes in you take a deep breath

[alerich]

I agree. This amp is running very cool. That's probably because of the junk tubes.

Am I understanding you correctly that the Tung-Sol EL34B is a junk tube? This would be the first place on the net where I have heard that comment. Some of the tube sellers (Tube Depot, for instance) think rather highly of it. Care to elaborate?

[sluckey]

Am I understanding you correctly that the Tung-Sol EL34B is a junk tube?

No, not at all. My comment has nothing to do with Tung-Sol. I hear that Tung-Sol is great.

I was simply agreeing with your analysis of the recommended cold bias setting by the manufacturer and 'your' statement about junk tubes as a possible reason for the cold bias...

I know that the original Sovtek tubes that this amp shipped with were junk and the original OTs were not robust. Maybe that accounts for the meager suggested operating conditions.

[alerich]

Got it. Makes sense. I have been tube shopping lately and did not want to overlook any input - especially here at Hoffman which I place a premium on.

Thanks!