tape recorder to amp conversion

[dmartn149]

Okay it's finished and working great, if anyone is interested. I mostly followed Adam Alpern's Micro Champ schem. I deleted the cathode bypass cap on the second half of the 12ax7 and didn't include a triode switch for the 6aq5. I also added a second channel that uses a 5879 for the first stage then feeds into the second half of the 12ax7. I used the schem from the 2nd channel of a Gibson ga90 as a guide for wiring the 5879.
It sounds really good. I think even better than my Epiphone Ea50 Pacemaker.


thanks again to all who offered advice.

[dmartn149]

You're welcome 470 Ohm (as in 5F1) for cathode resistor is good starting point, but, most likely you'll have to settle down with 250 Ohm for 6aq5.
Just measure voltage drop on it , do calculation of bias current and power dissipation and adjust it as needed.

The schem I'm using has that resistor @ 470 ohm. (see post #15) It's Adam Alpern's Micro Champ. I figured he's already done all the heavy lifting, so I planed to build it (mostly) like the schem.
There is more to re-purposing this old amp than I initially realized. Just the power supply kind of has me baffled.

[epis]

You're welcome 470 Ohm (as in 5F1) for cathode resistor is good starting point, but, most likely you'll have to settle down with 250 Ohm for 6aq5.
Just measure voltage drop on it , do calculation of bias current and power dissipation and adjust it as needed.

[dmartn149]

Thanks Damir. It is the PT and tubes that were in the recorder, so I'm sure you're right that it will work. I just didn't think to record the resistor values when I removed all the components from the chassis. I might just do as you recommend (build it like a champ) then check voltages.

[epis]

Ignore these high voltages you got with calculator. Your load isn't right ( 100k is almost no load).
If your tape recorder PT was configured to work with 6AQ5, you're good to go.
I would recommand to fallow completely 5F1 diagram and use same resistors values for B+ string for less hum and better screen protection for 6AQ5.
For cathode resistor use the value from tape recorder to start with.

Cheers, Damir

[Pdavis68]

I had it in my head that it was a 6bq5. You could run a 6BQ5 at those voltages (although they're beyond the maximums on the datasheets, but the Heathkit AA-161 ran the plates at 370V). I doubt you'd want to run a 6AQ5 at those voltages, though. The microchamp's transformer is about 80 volts lowers. That's a lot of voltage to bleed off with resistors, but you can, I suppose. I think I'll let someone more experienced than I chime in on this.

[dmartn149]

Okay, so I'm not sure I'm doing it right, but I measured 277v from ea. secondary to ct. and 230 ohms resistance. I plugged those numbers into the calculator. Am I right in assuming that the results I have to pay attention to are v(c1) v(c2) and v(c3) If I did the calculator right i have 374, 370 and 366 volts. That's too much for the 6aq5, right? So I need to plug in different resistors 'till I get the appropriate voltages? also, that last resistor (load) I entered 100k, the value of R9 in the schematic I posted earlier.

[dmartn149]

Thanks, I measured 562v, and downloaded the calculator. Not really understanding it all right now, so I will read the help file, probably have some questions, maybe learn something.

[Pdavis68]

R1 & R2 are connected to the cathode of the power tube instead of ground as this will raise the heater voltage by the 14VDC at the cathode, which will, in turn, reduce hum.

Yes, you'll need to figure out what the power transformer is putting out. You can measure it unloaded and then use Duncan's Power Supply designer app to figure out how to get the voltages to where you need (assuming it's high enough. I suspect it'll be good.)

[dmartn149]

I've stripped all of the components from the chassis that I won't be using and I'm starting to make a list of parts to order. I have a schematic for a "Micro Champ" I have a couple of questions (for now)
I don't know what the output from my PT is. Do I need to? how can I find out?
The schematic shows an extra power filtering stage (compared to gibson GA5) Is that really necessary or desirable?
Also shouldn't R1 and R2 be connected to ground? or to put it another way, shouldn't that lead that goes to C10 and R15 be on the other side of C10 and R15?
ThanksMicro-Champ-R2B.pdf

[dmartn149]

Well, It does, but according to the manual the line in is also the line out depending on weather it is in record, or play mode. Now that the chassis is out of the cabinet it's kind of hard to tell what mode the switch is in. It's a rotary switch that was operated by a rod and lever connected to the tape transport controls.

[olddawg]

Are you sure it doesn't have a like level out?

[dmartn149]

That might be an option to add while I'm converting it into a guitar amp. Would I just tap into the signal between the last coupling cap and the output tube? Or is there more to it than that? I don't think I want to try it right now. It still has all its original caps, and while it does amplify a guitar, it also squeals while doing so.

[olddawg]

Nope... Take the line out straight into the front panel input. Tape recorders by nature are pretty low noise and pack some gain. It's like a tube OD pedal.

[dmartn149]

Just for the hell of it... Before you do anything ... Try using it as an overdrive preamp into another amp. It might surprise you.

Do you mean sending the output from this amp into the FX loop of another? Before I did that I would at least have to replace the filter caps.