the meter is an avg. $40 Sperry DM. nothing fancy, but I like the simplicity. it's always seemed to work fine.
there were some mods done to this amp @ one point, & then they were "undone" as evidenced by 2 holes + several new solder joints, & a few replaced components. it's possible that something is amiss.
I printed out all of the schematics from your site & plan to check over the whole thing when I recap it. I'm so green when it comes to amps, but this will certainly be a good exercise
Try tacking in the 20k pot, and see if it helps. If not, you'll need the voltage doubler mod (or different output tubes). For example, the SED =C= EL34s seem to bias up fine in the stock Hiwatts I've worked on.
hey mark,
I've been using SED/=c= since the recent work & getting these numbers. tried the 20k pot this AM with no better results. are there any pictures of the voltage doubler circuit? I know you have the schem. on your site, but a photo always helps me as a beginner.
thx mark,
I found some online of others who breadboarded the circuit.
re: the quick & dirty - when I did this (w. a 20k pot) it only gave me about 4ma of variance. can I use a higher value pot (100k)? or add something else in line?
ok... so thru a series of trial & error, I kept increasing the value of the 47k resistor, until I hit the magic number 470k. the amp now idles between 38-40ma & plate voltage is approx. 470DCV.
so,
1. am I "allowed" to do this? seems to work, but is it OK?
2. if not OK, then why? is there a reason it happens to be precisely 10x the normal value? (I worked my way up from 100 > 220 > 320 > )
the amp seems happy now. I'm kind of happy (but feel like I need permission 1st).
I also tried increasing the 47k resistor to ground, and tried a pot. I could not get the bias idling lower than 38mA.
Then I bridged the 1K resistor on the other end of the bias circuit, and voila, biased dropped to 33mA keeping the 47K in place.
In other words, try replacing that 1K resistor with a 1K pot. That will give you much better adjustability than a pot in place of the 47K.
Edit: 38mA at 470V equals 17.85 Watt per tube for a max of 25 Watt. So youre on the 70% mark or close to max idle. That should be ok, but I'd prefer a little lower so the tubes will last a bit longer.
The reason the extremely high value works is that it is drawing virtually no current, and a transformer winding puts out a bit more voltage when the load on it is decreased. It appears the small difference in voltage was enough.
As long as there is some resistance (even high) in the circuit, it should function fine. It will just take a long time for that bias cap to discharge when you turn off the amp.
time to button it up. need to address a few minor cabinet issues, so the plinth/chassis is more stable. I think I'm going to load it w. the Weber FC12, as it's much lighter than the EV.
do I go w. the bias circuit? voltage doubler?
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