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rickl 4th January 2006 05:13 AM

found DIY amp - now what?
 
2 Attachment(s)
I got this mono 5881 PP amp from a friend. His dad built it in the early 60s. I traced out the schematic and attached it here.

Tubes include:
*really* cool GE 5V4G rectifier, 56-30, 188-5 date codes
2 Tung-Sol 5881 (v3, v4)
12au7, GE (v2)
12ax7, GE (v1)

PS: Stancor P-6014
OT: TALAO S-35A, wiring on side of tranny. 5000ohm input (CT), 16/8/4/g output

Before I bring it up, any recommendations? I have a variac so I'll be sure to bring it up slowly to help reform the electrolytics. Pull output tubes, bring up slowly with rectifier and driver tubes while monitoring B+?

A few comments on the schematic.

Each triode of V1 goes to a triode of v2. Common V1 cathode resistor (812 ohm measured) with a 40K/1W resistor in series to ground.

Each grid of v2 has a 1Meg + .01uF cap to the opposite pentode. Each v2 cathode is tied to the 4ohm and 8 ohm output taps.

The 4ohm tap has a 0.01uF paralleled with a 1.5k resistor.

Both V1 and V2 have 100k Rp.

Nice bias adjust/balance for pentodes. Common 250ohm cathode resistor that splits across two 100 ohm resistors to the pot. Pentodes have 3 and 4 tied (triode?) together.

Speaker leads are on the 4 ohm and 8 ohm taps. Ground and 16ohm taps are not connected to anything.

I hope everyone can read the schematic. I've never seen this kind of schematic before where 1/2 V1 drives 1/2 V2 with the diag feedback? to the output tubes. I searched the archives and couldn't find something similar.

Thanks,
rick
ps. I also got a Knight mono amp but I'll save that request for help later.

rickl 4th January 2006 05:15 AM

2 Attachment(s)
Here is a pic of the underside. It is very crowded but pretty easy to follow.

rick

SHiFTY 4th January 2006 09:32 PM

The 'diagonal lines' are indeed feedback, they go from the plate (output) of the driver tube to the input grid of that same tube, either to linearise them or to reduce the gain a bit. Possibly unneccessary, especially if you are going to use it as a guitar amp.

Firstly, check the base of the power supply capacitors, if they are bulged out and leaking don't use them. If they look ok but the caps get hot during use, replace them.

You should also check the 0.1uF coupling caps to the output tubes- if these are bad your output tubes will get red anodes (the outer grey shell) and die pretty fast. The tubes should be ok for starters, you certainly shouldn't replace any of them.

With the variac, I don't think you really need to go that slow- maybe 5-10 minutes until full voltage? Just keep an eye on the power supply caps and output tubes.

If you want to use the amp every day just replace all the capacitors and any out-of-spec resistors, it should run fine for years.

Also just in case you haven't already, please read the safety thread! High volts + fingers = bad juju. :smash:

rickl 5th January 2006 06:09 AM

Thanks SHiFTY,
PS caps look good. I didn't check the ripple on the scope. With only a CRC, I don't expect it to be very clean.

I replaced the .1 coupling caps with orange drops, I removed the 1M+.01uF feedback, added 12au7 cathode resistors (2k74+1uF), removed the .01uF||1k5 from the 4ohm tap and brought it up on a variac.

B+ settled to 266

6L6GCs measure:
pins 3,4: 331
pin 5: 0.86, 1.7 <--- ???
pin 8: 27.4, 27.4

One of the 12au7 triodes are not conducting (pin 3 = 0v).

Replaced with different 12au7, 4.38 volts and music!

Still will tweak a bit before I put in the original Tung-Sol 5881 tubes.

thanks again for your kind words,
rick


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