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Old 2nd June 2012, 05:06 PM   #1
cancon is offline cancon  Canada
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Default push-pull El34 bias question

Just a quick one here - I just biased an amp for my first time. For the cathode bias test all instructional posts say to break the ground connection between cathode and ground and replace with 1ohm resistor for every tube.

The amp in question is a Traynor YBA-1A (2x EL34) which has been previously modded with a trimmer pot on the bias supply and 1 ohm resistor on only one of the tubes. Instead of using one resistor per El34 as the guides instruct, the cathodes on both tubes are linked together and pass through a single 1ohm resistor to ground.

I set the current draw at 38mA, but since this value is for both tubes, what exactly is this reading telling me? I just wanted to ask before assuming things like two tubes = double the draw, and my values being off...

Thanks!
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Old 2nd June 2012, 05:18 PM   #2
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If the tubes are drawing equal current, they are each drawing 19mA.
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Old 3rd June 2012, 02:19 AM   #3
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The first amp I ever serviced was a YBA-1A. With a 540V B+ the plate dissipation is only about 10W per tube. The amp won't sound good there, it wants (needs) higher bias current. These amps came with a fan blowing on the tubes because they need it. Despite the fan, these amps eat tubes.
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Old 3rd June 2012, 03:00 AM   #4
cancon is offline cancon  Canada
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Plate current was 464vdc unloaded and 454Vvdc tubes installed.
But does the situation seem right? both cathodes tied together through 1ohm to ground...does the current represent both tubes (19+19?). The bias trim pot is about 75% through its swing, so I don't think there is enough headroom to hit 70ma.
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Old 3rd June 2012, 04:37 AM   #5
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There are four versions of the YBA-1A. The first 3 have a rectangular 17 inch chassis, the last is wider with a sloping front panel like a Fender Blackface. The first 3 use the same power transformer as the YBA-3, a monster with 800VCT secondary. An OEM version of the Hammond 278CX. The last version may have used a different power transformer, I don't have one in my collection. Note: I see you are from Canada. My experience is with amps sold in the USA, they may differ slightly from those you can find in Canada.

The bias supply is derived (don't drink and derive) from the HV secondary with a 2 Watt carbon composition resistor. Those resistors tend to increase in resistance with age and that reduces the bias voltage. Because the Standby switch is between the center tap and ground, that resistor sees about 1000V peak in standby mode. Replace with two or three flame proof resistors in series. The oldest version did not have a bias pot when it left the factory, so any bias pot was added later. If the bias pot does not have enough range, change the resistor in series with it and/or reduce the big resistor to the secondary.

Since there is only one 1 ohm current sense resistor, it measures the current of both tubes added together. You say the B+ is only 454V. That is strange. I have seen schematics of "Bass Masters" that have series connected filter caps, some with rectifier tubes. These amps are very old, before 1968. I've seen pictures of amps with four filter "can caps" mounted on the chassis. The amps I'm familiar with have two dual section 40uF 450V Mallory FP type chassis mount can caps in the corner near where the AC cord enters the chassis. An 80uF 450V axial cap is wired in series with the sections of the chassis mount cap to filter B+ in the YBA-1A. In the YBA-1 the B+ is only about 430V so the series cap isn't needed.

Does you amp have a convienience outlet on the back? I think that started in 68.
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Old 3rd June 2012, 04:47 PM   #6
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My ST70 has two tubes on the single bias current measuring resistor, and I have never burned out one tube and not the other. Mostly they just get tired and don't put out enough AC power into an 8 ohm resistor with good B+ voltage.
I do buy matched pair tubes, which gives you the advantage that the tubes have been warmed up on a test jig a few minutes before they measured them. Most construction problems in a tube will blow it in the first few seconds, so matched pairs get you longer life tubes. I've never had a defective one in my 8 or 10 pairs over the years.
19 ma per tube seems a little low. I've had to change the bias network resistors on the ST70 to get the proper 40-50 ma per tube with the installed pot. Of course, I'm no longer using a selenium rectifier, I'm using 1n4007 silicon diodes, for the bias supply. Not the same setup as yours, as sounds as if yours does not have a separate transformer winding for the bias voltage. But changing resistors to get the right idle current with proper B+ voltage is not unusual, I am saying. I use metal film resistors anymore, 2-3 watt versions for tube work (have 450 VDC rating). I'm buying mostly multicomp or vishay, they have been from india and thailand.
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Last edited by indianajo; 3rd June 2012 at 04:52 PM.
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Old 3rd June 2012, 08:09 PM   #7
cancon is offline cancon  Canada
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I am indeed up in Canada and quite close to the epicenter of the Traynor/Yorkville factories. I have never seen a sloped panel YBA before! must be rare, or a US thing??

My mistake - the unit in front of me is a YBA-1 (not A).
Schematic included with the chassis lists it as Jan '67.
There is a convenience outlet, and two Mallory FP type cans, each 40uf+40uf @ 450.
Solid state bridge rectifier, with the standby located at the ground-side of the bridge. The secondary HV line is not center-tapped. The bridge spills into two 40uf sections, (440V to the OT), followed by the choke, 3rd 40uf section, 5K resistor, and fourth 40uf section (375V to the plate resistors). The 5K checks out. The voltages I listed are printed on the schematic and as I said before, the plate voltages are 450V in practice...I've traced the lines and checked the components and I am sure all is working properly. In all my experiences with these amps they don't burn through tubes like crazy. The bias supply has a dedicated tap and its filters are new, as are the PS filters.

Seems the bias mod is simply a trim pot placed in series between the bias supply and R30/38K bias resistor to ground. I think the tech who rebuilt the bias filter network and added the trim pot made a mistake by leaving the original bias resistor at its factory value - so with the trim pot fully open, the lowest resistance is 38K, and this explains why I can't get the cathode current past 66mA. At this point the difference between 33mA per tube v.s. my target of 37mA is negligible and I'll just close the amp up and be done with it. My tubes are sourced locally from a reputable dealer and they are always very closely matched. I do see the merit of having each cathode separated though just to be sure!
Thanks for all the input.
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Old 4th June 2012, 12:06 AM   #8
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OK, a YBA-1 makes more sense. The very first ones had 7027A output tubes. Then they switched to EL34s and the full wave bridge power supply like the one you have. The date in the corner of the schematic is the original date of the drawing. There is usually a revision block on the drawing where changes are listed next to dates barely legible. That's a more realistic way to date the amp. If there is a six digit serial number on the chassis, the first two numbers indicate the year of manufacture.
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Old 6th June 2012, 04:00 AM   #9
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I like to bias EL34 at around 22 Watts dissipation but others suggest 18W.

With a plate voltage of say 460V that means:
18W => 39. 1 mA per tube idle current
22W => 47.8 mA per tube idle current

with a single bias setting for both tubes they will share idle current according to how well matched they are so I would go for the lower 18W value to allow some tolerence of out of balance tubes.

You therefore want to see (2 x 39.1) mA x 1 Ohm = 78.2 mV across the common 1 Ohm resistor. Just set it to 80 mV and be done.

You can try this a little higher or lower to suit your taste but don't go above the 22W per tube mark which would be 95.6 mV across that common 1 ohm.

Cheers,
Ian

Last edited by gingertube; 6th June 2012 at 04:02 AM.
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Old 6th June 2012, 04:28 PM   #10
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Don't forget when reading current across the a cathode resistor you are looking at both plate and screen current. So I would bias your amp to around 90ma which you should read 90mv across the 1 ohm resistor. About 10ma will be screen current for both tubes so you actually only have about 80ma of plate current, which is about 21 watts of plate dissipation. Svetlana specs for EL84 max dissipation is 25w, so 21 watts is about 85%.
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