I have been building an A1a wreck for the past couple weeks. I didn't rush and took my time to double check everything along the way. I'm using the transformer set from Heyboer. When I first powered it up, there was a loud lower freq. feeback regardless of the vol setting. I then plugged it in to my THD hot plate so I could get some voltage readings. The bias voltage was way out of whack (-12.6 vdc) and it was cooking my power tubes. I did manage to get a B+ reading....it was low around 365 vdc. My guess was that the excessive current demand from the power tubes is causing the low B+ reading. Some observations: the feedback did not go away when the first two tubes were pulled. It went away when the PI tube was pulled. I do know the power amp tubes are good because they came out of a working amp with no problems. I changed all the 12ax7's twice so I don't see a tube problem but I don't have a clue why the bias voltage is screwed up and the adjustment pot doesn't do much at all. I checked the bias pot operation and the bias circuit for errors but didn't find any. Also, It did blow a couple fuses during all of this. Anyone have any advice on what to do next?
Hi,
I would check the cap in the bias supply,if it's in backwards you would get the low bias voltage.If it was in backwards,I would replace it.
keep us posted!!!
Chris
I looked at the bias caps and they are hooked up as follows...positive lead to ground and negative lead to bias circuit path. Is there anything I'm missing on the schematic? (like special grounding requirements for the bias circuit)
The oscillation you're describing may be because the output transformer is reversed, causing a positive feedback loop rather than negative. Try inverting the phase by either reversing the leads from the coupling caps to the output tubes or reversing the leads from the output transformer to the plates of the power tubes.
I'm fairly sure I know what your bias problem is, and I really should have brought this up sooner, but I think the flaw is in the schematic diagram itself. I don't calculate anywhere near an adequate bias voltage from the circuit in the A1a schematic. Has anyone used this and been sucessful? With a 300-0-300 transformer, I'd modify the circuit by changing the 220k to a 100k resistor, swapping the places of the 20kB pot and 47k resistor, and changing that 47k to a 27k instead. That should give you a range of about -46 to -26 volts, good for EL34's. If the above wasn't clear enough, I can draw it up for you. That 100k should be rated for at least 2 watts and you can get there with four half-watt ones wired in series-parallel if you don't have the right rating.
Make sure you fire the amp up without tubes to check your bias voltage, that way you won't blow any up testing it!
Thanks for the info. I too calculated too little bias voltage using this circuit. Surely I'm not the first to get bitten by this...anyone else had this problem? I just hope I haven't fried anything. Also I may have the dreaded positive feedback problem.
I'm also surprised this is the first time anyone's mentioned having any problems with the bias circuit in A1a as well! Once you get a nice solid bias voltage and it'll fire up safely (start with the most negative voltage setting), you can check the feedback issue by just shorting the outer lugs of the presence pot together with some test clips; the output won't care about the 100k resistor being effectively across it, and that way you don't have to waste your time desoldering anything if it's something else (though the feedback does sound like a likely culprit). Just be sure you don't short a point before the fb resistor to ground in doing this, especially if you've got it mounted directly on the presence control. Also, having just re-read your very first post, I wanted to make sure that you've got a load (speakers or resistor/hotplate) on the output whenever the power tubes are installed and the power's on, right?
Here's that bias mod for anyone else having problems:
Sorry I didn't see your question in your last post. Yes I'm using a THD hot plate in the 8 ohm load mode. That way I don't have to hear all the feedback.
I did the bias mod suggested by Matt and it worked great (thanks so much). I was able to get a range of -42 to -22 volts. I also reversed the phase on the OT and that took care of the feedback squeal. My very early impression of the amp is, it's loud and quite bright but it has a nice classic overdrive tone (gotta love that power amp distortion). The only issues I have at this time are splatter at high bass settings and a small amount of hiss. Overall, the amp is very quiet and well behaved. Now, what bias should I run the EL34's at (in milliamps please). Thanks to everyone for helping me get the amp going.
Before I forget.....with all the people I have seen talking about building the A1a version, I am floored nobody has ever complained about the bias circuit. It makes me wonder how many actually complete their build. Oh well, I'll just update my schematic.
I'm glad to hear it worked out well. To bias your amp, and in this I'm assuming your plate voltage is about 410V with EL34s, you can adjust the bias to be anywhere up to about 42mA per tube (the ~70% plate dissipation limit). What I'd suggest would be to start at about 30mA and play a little bit. Once you get a semblance of what it sounds like at that bias point, increase the current flow by a couple of mA's and compare the tone. Keep increasing it like that until you either don't notice the sound is getting better, or you hit that 42mA limit. People often say to just bias the amp at 70%, but really there's no reason to run it any hotter than it has to be for good tone. Don't be worried if it sounds good at 50%, especially as you'll benefit from better tube life.
I'm surprised as well by the flaw in the circuit, but I think it's not so much that people don't finish their builds as it is that they often don't follow the bias circuit verbatim. It's pretty common to have to adjust something in the negative supply when using different transformers, etc. so a lot of folks probably just shrug off the difference.
Thought I would throw a couple pics up of the amp in this thread. Also got some voltage readings with the amp biased at 40ma. They seem a tad low what do you guys think?
I also took a quick reading with out the power tubes and B+1 was 410.
Eric
Forgot to mention, please forgive me for the cheesey radio shack knobs these are just temp. until I build the cabinet. Me and a buddy will try and get some sound clips laid down next weekend.
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