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Topic: 66 Montgomery Wards Valco amp - 6L6GC cathode bias advice |
Bill Sinclair
From:
Waynesboro, PA, USA
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Posted 27 Oct 2015 7:26 am |
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Sorry for the long-winded post but here goes: For several years I've drooled over the old 2X10" Supro amps that show up on ebay occasionally. When I had the opportunity to buy this Valco built Mongomery Wards amp for about 1/3 what the Supros go for,I decided to give it a try. Model 62GVC 9063A. I've been needing a slightly louder two channel amp for harmonica and lap steel. My 1948 Silvertone works pretty well for most gigs but I wanted a little more power and a reverb channel for the lap steel.
Here's the original auction: http://www.ebay.com/itm/VALCO-VINTAGE-ALL-TUBE-COMBO-PSYCHODELIC-GRILL-CLOTH-/221866655348?hash=item33a8487e74:g:6~0AAOSwyQtV4Hz6
Replaced most of the electrolytic caps and a couple of errant resistors. It sounds pretty good but I'm a little confused about cathode biasing the 2 (RCA)6L6GCs. The original cathode resistor is a 200 ohm with a parallel 33ufd bypass cap. I figured that since the voltage at the wall is at least 10% higher than it was 50 years ago, I should bump it to a 250 ohm. Can't say I liked the result. Diving into it a little more on the internet I'm finding that cool for fixed bias is downright cold for cathode bias. I have an industrial electronics background but I've got a lot to learn about tube amps. With the original resistor back in the circuit I've got 377 VDC plate to cathode and 120mA through the cathode resistor (60mA per tube, assuming they're matched). About 23 Watts of plate dissipation or about 75% according to the robrobinette.com bias calculator. Does that sound like a good place to leave it? With a pair of Sovtek 6L6WGC tubes it runs more like 87%. Some of the internet guys say to run cathode bias close to 100%! What am I gaining/losing by running it hotter? As is often the case seeking info on the internet, I feel like my head is going to explode with differing and/or unclear info. The schematic, if you're inclined to look it up, is nearly identical to the Supro S6651 except that it uses a 6973 pentode for a reverb driver instead of the two triodes of a 12AX7. http://www.schematicsunlimited.com/s/supro
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John Limbach
From:
Billings, Montana, USA
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Larry Jamieson
From:
Walton, NY USA
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Posted 27 Oct 2015 11:52 am
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I don't know much about electronics, but that is
some wild grill cloth. I Like it! |
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Bill Sinclair
From:
Waynesboro, PA, USA
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Posted 27 Oct 2015 12:24 pm
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John,
Thanks, I should have known that Uncle Doug would be a good source. I love his laid back videos. I'll watch it this evening.
Larry,
Yeah, me too. The crazy grill cloth makes up for the clunky design of the cabinet. It's lighter than it looks - only slightly larger and about the same weight as a Deluxe Reverb. Appears to be a fairly rare amp. Not necessarily desirable - but rare!  |
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Bill A. Moore
From:
Silver City, New Mexico, USA
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Posted 27 Oct 2015 2:25 pm
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| Bill I have one from the same era, albeit, a SE design with a 12. Looking at the pics, I remember troubleshooting mine, true "point to point" wiring! Also isn't it amazing what a great reverb comes out of those little Hammond cardboard boxes! |
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Bill Sinclair
From:
Waynesboro, PA, USA
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Posted 27 Oct 2015 3:30 pm
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Bill,
Yes, a lot of people seem to disparage the cardboard reverb but I think it's going to be just fine for my purposes. Tremelo sounds great too but I don't use it.
I just watched about an hour of the Uncle Doug tutorials on youtube but he kind of botched his explanation of cold vs. hot bias (in his Single-end biasing video). He tried to correct his graphs after the fact but it made for a pretty confusing tutorial. Still love his amp repair vids. I'll keep digging. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 27 Oct 2015 9:28 pm
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The thing about self-bias... if the voltage goes up, the bias goes up. I'd be happy with where you are. Replacing the cathode bypass cap is normally a good thing if you haven't done that. Those Sovtek 6L6's like it hot... you should be good to go.
_________________
New FB Page: Lap Steel Licks And Stuff: https://www.facebook.com/groups/195394851800329 |
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Bill Sinclair
From:
Waynesboro, PA, USA
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Posted 28 Oct 2015 4:58 am
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"The thing about self-bias... if the voltage goes up, the bias goes up."
Thanks Stephen, that makes sense. I've run across a figure of 70% plate dissipation for fixed bias that a lot of people seem to shoot for. Is there a similar rule-of-thumb figure for cathode biasing? I guess it depends on whether you're shooting for class A or AB operation but my grasp of that whole issue is somewhat tenuous. |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 28 Oct 2015 7:40 am
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| Bill Sinclair wrote: |
"The thing about self-bias... if the voltage goes up, the bias goes up."
Thanks Stephen, that makes sense. I've run across a figure of 70% plate dissipation for fixed bias that a lot of people seem to shoot for. Is there a similar rule-of-thumb figure for cathode biasing? I guess it depends on whether you're shooting for class A or AB operation but my grasp of that whole issue is somewhat tenuous. |
The deal with biasing... your amp was designed to be biased at a certain idle setting. This bias actually changes the impedance of the final amp, so there definitely is a 'sweet spot' where the tubes are happy and the output and power transformers are happy.
If you want to approach this scientifically I'd connect a variac and power the amp down to 117VAC, then measure the % dissipation, then remove the variac and see if that % dissipation changes. It definitely does on a 'fixed bias' (read: uses bias voltage) amp... I haven't done this experiment but I'm fairly certain that cathode-biased ('floating bias') amp is more immune to this change.
_________________
New FB Page: Lap Steel Licks And Stuff: https://www.facebook.com/groups/195394851800329 |
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