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previous: Marc MB;What are you using to dr... -- 3/14/2000 3:56 AM

Re: how do you draw the schematics?

3/15/2000 1:58 AM
MBSetzer	Re: how do you draw the schematics? Well, looks like some more comments on this oddball circuit. I did correct the drawing and added the voltages, so throw the old one away and the replacements are davvs106.pdf & davvs106.gif at http://members.aol.com/mbsetzer I am using Corel Draw 7.0 then exporting it as a GIF or PDF file, neither one looks quite as good as the original CDR file, but it seems to work OK. If I was really serious I would use real schematic capture so I could simulate using PSpice. But I'm not that advanced yet Plus since I've got the real amp it is easier & quicker to just take measurements. As usual Ken's insight is seeming correct to me, that 2.2Meg on the lower EL84 is high enough to make the power tubes less stable than they should be, and the Negative Feed Back through the 35K helps lots. When I removed the 35K it became a serious effort to stop oscillation. This screen follower drive for the lower EL84 is a great oscillator too. Anyway the first incorrect drawing makes me think it's best to try to get actual reality to be one step ahead of virtual, so now that the drawing is a realistic representation, actually by now the amp has been modified majorly to work with guitars. I'll update that soon and it will be different, but including the self-inversion and exteme efforts to tame the beast. But now a little elementary circuit analysis of the stock amp from my point of view: This is intended to be cheap & reliable as well as remain on all the time. The OPT is a full size bigger than a Fender Deluxe OPT which is for two 6V6's, but this just has 6BQ5's. It does work apparently well with one 6BQ5 removed. The lower one, since if you pull the upper one there is no connection from the preamp to the OPT So there may have been some allowance for unbalanced operation, I'll discuss later. It may be looked at like mainly a mixer for the PA Mic & the Program Music through the 330K's to the second half of V1. The Mic will need the extra gain stage to bring its very weak signal to the level expected of the phono input. Of course there is no RIAA network so I really think the PHONO label for this input was carried a few decades too far. Looking at the 100ohm cathode resistor on V1b and its minuscule 0.06 volts of cathode bias you realize that V1b is intended for small signal inputs only. I was easily overdriving this stage with the guitar plugged into the MIC input. The voltage gain of V1a is 50, a 0.5Vp-p Mic Input results in 25Vp-p at pin 7, a maximum of 3Vp-p can be handled for relatively linear operation, resulting in 150Vp-p maximum from pin 7, but this would never be reached by the micrphones this was designed to use. For V1b the voltage gain is 32, it can only handle 0.25Vp-p at pin 5 and output 8Vp-p maximum from pin 6 before clipping. Looks like it might have been well matched to remain within the linear operation of V2 even when clipping. Of course if you put a clipped signal into V2 you will get a clipped one out, but you won't actually be overdriving V2 to get it. Then come the power tubes. V3 is working slightly harder than V2, and after I added 1K to the 100ohm Rk of V1b, it was possible to pass a large enough clean signal to V2 to clip its output. This clipped output would also appear at pin 9 and is the source for the inverted input to V3. With the power section as-is, the clipped peak-to-peak (peak-to-flat?) signal excursion driving V3 was about as high as the clean p-p input to V2 from the preamp. But the waveforms are different, one symmetrical and not the other. This would seem to be intentional, maybe allowing voice intelligibility with somewhat balanced power from both 6BQ5's at reasonable volumes. Not a hifi amp. When the signal is turned down enough for the input of both 6BQ5's to be undistorted, there is about a 2:1 ratio of input to V3 compared to V2. That would be a little quiet to hear an announcer and you might miss that blue light special Kmart shoppers Probably not bad for background music, to be overwhelmed when the mic was keyed. Now that it's a guitar amp its characteristic sound is asymetrical distortion even after soldering dozens of components in & out of it, it more or less rocks. More than I ever thought it could but less than perfect. So more drawings after I finalize the circuit, Rebel if you are patient you might want to modify to my upcoming spec, plus it could be considered a generic diagram, wiring for a 12AX7 instead of the 6EU7, and some other more common rectifier would be no big difference either. With this hardware and tube set it seems to be moving beyond Pro Jr. territory while it's still on the bench. It is getting fun and I'll try to get another sound clip too. Also, just remembered, maybe there are still errors on my drawings since I say 1 M A for the pots when I mean Audio Taper. Looking at teds posting recently the A taper is apparently linear, somebody correct me if so, and maybe see if this is consistent with the Fender BF & SF drawings. Mike

3/15/2000 1:58 AM

MBSetzer

Re: how do you draw the schematics?

Well, looks like some more comments on this oddball circuit.

I did correct the drawing and added the voltages, so throw the old one away and the replacements are davvs106.pdf & davvs106.gif at

http://members.aol.com/mbsetzer

I am using Corel Draw 7.0 then exporting it as a GIF or PDF file, neither one looks quite as good as the original CDR file, but it seems to work OK. If I was really serious I would use real schematic capture so I could simulate using PSpice. But I'm not that advanced yet ;-)

Plus since I've got the real amp it is easier & quicker to just take measurements.

As usual Ken's insight is seeming correct to me, that 2.2Meg on the lower EL84 is high enough to make the power tubes less stable than they should be, and the Negative Feed Back through the 35K helps lots. When I removed the 35K it became a serious effort to stop oscillation. This screen follower drive for the lower EL84 is a great oscillator too.

Anyway the first incorrect drawing makes me think it's best to try to get actual reality to be one step ahead of virtual, so now that the drawing is a realistic representation, actually by now the amp has been modified majorly to work with guitars. I'll update that soon and it will be different, but including the self-inversion and exteme efforts to tame the beast.

But now a little elementary circuit analysis of the stock amp from my point of view:

This is intended to be cheap & reliable as well as remain on all the time. The OPT is a full size bigger than a Fender Deluxe OPT which is for two 6V6's, but this just has 6BQ5's. It does work apparently well with one 6BQ5 removed. The lower one, since if you pull the upper one there is no connection from the preamp to the OPT ;-)

So there may have been some allowance for unbalanced operation, I'll discuss later.

It may be looked at like mainly a mixer for the PA Mic & the Program Music through the 330K's to the second half of V1. The Mic will need the extra gain stage to bring its very weak signal to the level expected of the phono input. Of course there is no RIAA network so I really think the PHONO label for this input was carried a few decades too far. Looking at the 100ohm cathode resistor on V1b and its minuscule 0.06 volts of cathode bias you realize that V1b is intended for small signal inputs only. I was easily overdriving this stage with the guitar plugged into the MIC input.

The voltage gain of V1a is 50, a 0.5Vp-p Mic Input results in 25Vp-p at pin 7, a maximum of 3Vp-p can be handled for relatively linear operation, resulting in 150Vp-p maximum from pin 7, but this would never be reached by the micrphones this was designed to use.

For V1b the voltage gain is 32, it can only handle 0.25Vp-p at pin 5 and output 8Vp-p maximum from pin 6 before clipping. Looks like it might have been well matched to remain within the linear operation of V2 even when clipping. Of course if you put a clipped signal into V2 you will get a clipped one out, but you won't actually be overdriving V2 to get it.

Then come the power tubes. V3 is working slightly harder than V2, and after I added 1K to the 100ohm Rk of V1b, it was possible to pass a large enough clean signal to V2 to clip its output. This clipped output would also appear at pin 9 and is the source for the inverted input to V3. With the power section as-is, the clipped peak-to-peak (peak-to-flat?) signal excursion driving V3 was about as high as the clean p-p input to V2 from the preamp. But the waveforms are different, one symmetrical and not the other. This would seem to be intentional, maybe allowing voice intelligibility with somewhat balanced power from both 6BQ5's at reasonable volumes. Not a hifi amp. When the signal is turned down enough for the input of both 6BQ5's to be undistorted, there is about a 2:1 ratio of input to V3 compared to V2. That would be a little quiet to hear an announcer and you might miss that blue light special Kmart shoppers ;-)

Probably not bad for background music, to be overwhelmed when the mic was keyed.

Now that it's a guitar amp its characteristic sound is asymetrical distortion even after soldering dozens of components in & out of it, it more or less rocks. More than I ever thought it could but less than perfect.

So more drawings after I finalize the circuit, Rebel if you are patient you might want to modify to my upcoming spec, plus it could be considered a generic diagram, wiring for a 12AX7 instead of the 6EU7, and some other more common rectifier would be no big difference either. With this hardware and tube set it seems to be moving beyond Pro Jr. territory while it's still on the bench.

It is getting fun and I'll try to get another sound clip too.

Also, just remembered, maybe there are still errors on my drawings since I say *1 M A* for the pots when I mean Audio Taper. Looking at teds posting recently the *A* taper is apparently linear, somebody correct me if so, and maybe see if this is consistent with the Fender BF & SF drawings.

Mike

Replies:

MBSetzer Davis106 mp3's -- 3/20/2000 2:27 AM
Steve A. Re: how do you draw the schematics? -- 3/20/2000 8:26 AM