Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Cups on September 17, 2011, 04:20:30 pm
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Just got back a broken Traynor YBA-2a (the cute little 15 watter with el84)
When I first repaired it I had to replace the OT. The plate socket at V1 blew and melted the socket. I installed 1k screen resistors and put in some 1R current sense resistors.
Amp came back sounding harsh with barely any volume. Screen resistor came loose with a little fiddling and I noticed scorch marks on the resistor. Shope owner said the guy has the amp pegged all the time with a booster in front.
Replaced the screen resistors. Replaced all 'lytic caps (except for bias wich I'm getting to) and the harsh sound is gone but the volume is about the same.
- Voltages are about 30V higher then schem on plate and screen (430 instead of 400)
- bias current is between 33 -38ma
- changed output and pre tubes
Anything jumping out at you guys?
I'm going to change the OT anyway but a lot of times, and from my (limited) experience, changing something big is often unnecessary.
Thanks for any advice.
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Is this the amp?
http://community.webshots.com/photo/fullsize/1334395118045125817ZuofSv (http://community.webshots.com/photo/fullsize/1334395118045125817ZuofSv)
If it's fixed bias, this obviously is not the right amp. <comment about bias capacitor?>
If you are running EL84s with 430 on the plates and 38ma of plate current per tube, then you are WAY over maximum rated plate dissipation of 12 Watts.
430 * .038 = 16 Watts = OUCH!
Do you mean a total of 38ma through the cathode resistor? If so, that means it's actually biased really cold.
Bottom line, we need more voltage data and the right schematic to help.
Tube data sheet:
http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL84.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL84.pdf)
See "Limiting Values" on third page. Note 300 Watt max plate voltage.
Even if you intended 330 volts, it's still at 12.5 Watts. A starting point usually is 70 % of maximum plate dissipation or 12 * 70% = 8.4 Watts. That would translate into 25ma of plate current. However, the "Operating Characteristics" for Push/Pull with 300 volts on the plates works out to almost 90% of max plate dissipation.
Have you measured the value of the power tube cathode resistor? If it measures the stock 250 ohms, maybe you need to try a bigger one. If your amp is fixed bias, you need a higher (more negative) bias voltage. This assumes 38ma PER TUBE again...
Hope that helps,
Chip
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Modern "EL84" will give useful stage-amp service at 400V, even 430V.
They won't last forever.
I don't see where you put new EL84s in??
Clean the heck out of the sockets. That was half the problem with my vintage BassMate.
> A starting point usually is 70 % of maximum plate dissipation
For FIXed-bias.
EL84 was sold for "simple" amps, which are self-bias, and typically run 90%-110% of Pdiss rating.
7189 is the same tube, rated for 400V and specced for fancy-pants fixed-bias service. Maybe in 1956 the tube factory had a cheaper plate-stuff and a high-voltage plate-stuff; but they used hundreds of pounds a year of plate-stuff. Nowadays the total market is dozens of pounds, and they probably use a "better" grade for every model.
The idle current on the 7189, and on the Traynor, can be very low, 15mA per tube. Crossover distortion is not large. Anyway "pegged all the time" means it never sits at "idle".
> the guy has the amp pegged all the time with a booster in front.
This may not be the best amp for such use/abuse.
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Just redid the whole bias setup, and added adjustable individual bias pots. It is fixed bias.
Here's the schem: http://www.lynx.net/~jc/720606_YBA2_AB.gif (http://www.lynx.net/~jc/720606_YBA2_AB.gif)
Now I don't have enough negative voltage. :BangHead:
I'll figure this out. Hopefully without blowing anything up.
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Can you show us exactly what you did to the bias circuit? In the original circuit, you can decrease the value of R25 and/or increase the value of R27 to increase the negative bias voltage. The same should apply to your modified dual bias circuit, unless you did something unconventional or made a wiring error.
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I changed r25 to 130k
upped the first cap value to 16mf (i don't have any 8mf of that voltage)
replaced r 27 with 25k pots in parallel with the wipers going to the 100k (r23, r22)
I have a 10k to ground between the pots.
r26 was changed to 12k
This is embarassing but I had the caps in backwards to. Seems I'm a little too used to putting the neg to ground.
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I changed r25 to 130k
You would need to decrease R25 value from 120K to increase the negative bias voltage.