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Anyone for upping the resistor 8.2k on the plate supply that goes to the 30uf 600v cap ? I got 767vdc there and that 8.2k only drops it to around 624 volts which is way to high for V1's 12DW7. I put around 25k and it dropped all the voltages to the right values. It had a 4.7k and the 12DW7 was toasted. Just wondering if anyone else has seen this on original SVT's ?
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KB -
I got 767vdc there and that 8.2k only drops it to around 624 volts
624V wouldn't be safe for a 600V cap either. I don't recall ever seeing 767V on an SVT before. Are you reading the voltage with all the tubes installed and the amp properly biased? Has the PT been replaced? The 4.7K 15W resistor was stock in the earlier SVT's that had 660V at the plate supply. Later SVT's had 695V on the plate and used the 8.2K 15W. Have you checked your voltages with another meter for verification? Any time I get readings that are way off from the norm I suspect something wrong with my measurement procedure first, then I look at the circuitry. If your power tubes aren't pulling current, that would raise the B+ voltage. Check all the 5W plate resistors and screen supply resistors and diodes.
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Actually Rick ,at the time only two tubes were in and yes I'm suspecting it's the older model but I don't think the tranny was changed as it has the original numbers on it. I'll install the rest of the tubes and check it with my other meter but it was biased at 38 mv across the 1 ohm cathode resistors. I've checked every diode and resistors in that amp and they all test good. I did a cap job and when variaced it up is when I noticed the high voltage on the plates. The V1 12DW7 on the power side was toasted which was causing other voltage variations and slamming the signal on the preamp side. When I replaced the 12DW7 it straighted the triode voltages out but the plate voltage was still 767. What do think about that high of a dropping resistor of 25k ? BTW Do you like those 12DW7's as guitar amp tubes ?Originally posted by Rick Erickson View PostKBComment
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Amp Kat,
I think once all the 6550's are installed and the bias is set to factory spec - or thereabouts - your plate voltage will likely drop down to near 700V. 25K seems a bit large to me, but if that's what you end up needing to get the voltage where you want it then that's what you need - but you need to test the amp under actual operating conditions. For the record:I have never cared for 12DW7 tubes in SVT's or any other Ampeg amp. I'm not saying there couldn't be a good one somewhere, but I have seen & replaced a LOT of bad ones, and had problems with them right out of the box*. I always recomend replacing them with 12AX7's when I work on an older SVT. When I recently worked on a couple of The Rolling Stone's SVT's I noticed they had already been converted.
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*Not to say I haven't had problems with new 12AX7's - just that they are more plentiful and the chances of finding an occasional good one are a bit better.Comment
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Yeah I'm with you on the 12ax7's and that's exactally what I replaced the 12DW with. Glad to see you fixed those old boys up and when you hear those old bands you can usually pick out the ones that used the SVT which was almost all of em as they just have that tone. We'll see on those voltages when I get the resistors in. 25k 15 watters aren't something I normally stock ya know. Thanks again Rick,KBComment
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Well I did it Rick and just as you said when I put all the tubes in it and biased it up the plate voltage was 706 and pretty steady. I used the 20k 20 watt and that thing is getting pretty damn hot and has me worried a little but like you said it's what I had to do. Any clue as to why that voltage is so high. I checked the HV winding and it was right on calculation subtracting the 1.414 number so ? One more thing is the calibration procedure says to bias it at .072 . Is that one tube or two ? At .072 mv would put the 70 % mark of a 6550 of 42 watts at 29.4 and divided by .072 would be 408 of plate voltage. Numbers don't jive ?KBComment
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Kerry,
These amps are designed to be biased at 72mA (.072V meter reading) per 3-tube push/pull half, or 24mA per tube - a bias point of roughly 40% combined plate/screen dissipation. IME the amps sound excellent at this setting; loud, clean, and dynamic.
IMO it's also really important to adjust the AC balance control, using a sine-wave signal at maybe 200W output or so.
RayComment
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20k 20 watt and that thing is getting pretty damn hot
Read the voltage across the resistor and calculate the power it is dissipating. The schematic shows 210V (695V-485V) across an 8.2K resistor for a dissipation of ~5.4W. They used a 15W resistor to ensure that it didn't burn up. As long as your dissipation is less than 1/2 the rating of your resistor you should be fine. A 20W resistor is going to heat up in normal operation, that's what it does - converts energy into heat. Follow Ray's advice on biasing and everything should fall into place. Have you found the bias test points that sit behind the grill in the front of the power amp? That's where you should be measuring & setting your bias & Balance, with all tubes installed and the incoming line voltage at a solid 120V, assuming you are in the USA.
REComment
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Personally, I like the sound of the 12DW7 in these SVT's, and they aren't THAT hard to find. NOS is still available and not too expensive, and JJ makes a pretty good one. The 12AU7 half of the 12DW7 provides better drive than a 12AX7 would. Oh well....each to his own...Comment
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Yes Rick I did find the bias controls and everything else on this amp as it was totally taken apart. The controls or meter jacks labeled K1 and K2 are used. K1 sets the bias in mv which is where it says to put it at.072 and K2 gets set to zero. (bias-balance). K3 is the balance control and Ray I will do the sign wave method on it this afternoon. Hum balance is heater related and pretty much a matter of ear as I set it under my flourescent lights. Ok fellowsthanks a bunch as this ought to take care of the old fellow. This amp is loud as hell and sounds excellent. Guess your right about the tubes SG and it's just a matter of what works best for you and yes each has it's own
KBComment
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