Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Deric on October 28, 2018, 01:08:05 pm

Title: Rickenbacker Model 98 Schematic Check
Post by: Deric on October 28, 2018, 01:08:05 pm: Working on a Rickenbacker Model 98 and can find no info anywhere. Google turned up a couple of pics but that's it. Amp works but has very low output. Drew up a schematic but would appreciate input from anyone willing to look it over. The tubes shown are what was in the amp - no idea if they are correct but no reason to believe they aren't.

Will replace power cord and filter caps. Plate resistors on the 12ax7 are both way off. ~1M or more so will replace those too and go from there.

Thanks!!
Title: Re: Rickenbacker Model 98 Schematic Check
Post by: Deric on October 28, 2018, 01:13:25 pm: Pics...

https://photos.app.goo.gl/ktQggCckmPDvBERv6
Title: Re: Rickenbacker Model 98 Schematic Check
Post by: PRR on October 28, 2018, 01:47:36 pm: > Drew up a schematic but would appreciate input from anyone willing to look it over.

All looks reasonable; and a nice job for a back-trace.

The 2nd half of the 12AU7 is a cathodyne; my own tiny mind likes to draw top-to-bottom to see the flow so I rotated some parts to check that it really was what I thot it was.

> very low output

Low gain (have to work too hard to get 30 Watts), or low maximum power (like 5 Watts max)?

In any case, voltage readings are a next step. We'd also like to know if both big bottles are pulling a share; since they are common bias the best way may be to put in other tubes and see if cathode voltage changes much. (5881 are high-spec 6L6; 6L6GC are hi-spec 5881; today any "6L6" will be fine in this plan.)
Title: Re: Rickenbacker Model 98 Schematic Check
Post by: Deric on October 28, 2018, 10:47:25 pm: Thanks for taking the time to look it over!!!

No tubes installed (besides 5y3)
B+1 455
B+2 409

With all tubes
B+1 356
B+2 267

V1
1 51.6 vdc
2 -0.9 cdc
3 0.0
6 76 vdc
7 -1.027 vdc
8 0.0

V2
1 47 vdc
2 0.0
3 2.25 vdc
6 207 vdc
7 47 vdc
8 56 vdc

v3
3 346 vdc
4 266 vdc
5 4.2 vdc
8 21 vdc

V4
3 344 vdc
4 266 vdc
5 0.5 vdc
8 21 vdc

Swapped V3 and V4 and numbers stayed the same. Did not try any other tubes.

EDIT: Just realized V1a on the schematic has the pins numbered wrong.... Voltages listed are for correct pinout...
Title: Re: Rickenbacker Model 98 Schematic Check
Post by: terminalgs on October 29, 2018, 10:51:09 am: Quote from: Deric on October 28, 2018, 10:47:25 pm
v3

5 4.2 vdc

V4

5 0.5 vdc

4.2vdc? should be 0vdc. check coupling cap, seems to be leaking DC.
Title: Re: Rickenbacker Model 98 Schematic Check
Post by: PRR on October 29, 2018, 12:35:03 pm: Agreed. Not odd to find 0.5V on a power tube grid. We want grid voltage to be much less than grid-cathode voltage, at least factor of 10. 21V bias means we want less than a couple volts; really less than that. 4V is too much.

Cuz it is easy, swap the 6V6es and see if it follows the tube. If it stays with the socket, remove the 0.05u there and see if it comes down to a part-Volt.
Title: Re: Rickenbacker Model 98 Schematic Check
Post by: Deric on October 29, 2018, 01:31:52 pm: Did swap output tubes with no change. Not looking forward to pulling the circuit board... appears all parts just run through holes then connected to each other underneath.

Have parts coming today to get started.

Thanks!
Title: Re: Rickenbacker Model 98 Schematic Check
Post by: Deric on November 28, 2018, 05:56:38 pm: Got this buttoned up - sounds great. Replaced filter caps and ended up just rebuilding the main board. Fired up with no issues. V1 was microphonic but all other tubes were fine. Cool little amp. Link to pics above has been updated.

Thanks for the help!

Updated voltages for reference:

With all tubes
B+1 355 B+2 285

V1
1 100
2 -0.9
6 134
7 -1.5

V2
1 44
2 0.0
3 2.0
6 233
7 44
8 55

V3
3 348
4 284
5 .015
8 21

V4
3 348
4 248
5 .012
8 21