SHFA1 recap tips

Hi I've just finished recapping a Seeburg TSU1 and it's now time to turn my attention to the SHFA1 amp.
All caps are original and require replacing, compared to the TSU this looks a real bitch to recap.
As per the TSU1 is there any mods I should be performing to make this amp more reliable?
Any tips or hints on recapping methods most appreciated.

Cheers

John

While not mods, it is important to replace the selenium rectifiers in the bias and AGC circuits. I used whatever I had with at least a 100 volt rating at around 1 amp or more.

i don't envy you in having to rebuild an SHF1 they were unnecessarily complex -especially in the bass tone control ckt. The amp used in the Q was much easier and IMO better sounding.

Bear in-mind that beginning with the K and through the Q Seeburg used a lot of feedback around the two 12AX7 stages in the treble circuit and this makes values in this loop more critical than otherwise. The tiny ceramic caps in this area actually tune the bass curve quite radically. I'll look at my notes from April 1994 when I tested the response of these amps, but I do -slightly- increase one of the ceramics to move the bass boost peak down from 100Hz to a fatter and less boomy 72 Hz approx. This is how all the 201 amps I have out are voiced to this day.

The original preamp used in the stereo amps is a bit better than the single stage used in the K-201 but still less than adequate -especially if lower output styli are used or if you replace the tonearm-pickup with the later type (as I did). On the stereo machines I bypassed the original pre with an external modern packaged preamp.

Rob

Thanks for the reply Rob look forward to seeing what changes you recommend.
Can you confirm that the component in the picture attached is in fact a diode CR101?

Yep. that is a diode --don't know which one, but it should be replaced in any case.

Rob

FYI--The red dot indicates the + side of the diode. Ron Rich

Ron the red dot was on the cathode side of the diode, I've allways thought of the positive side being the anode side?
I have replaced the two diodes as per the drawing below,
3/4 of the way through the recap this must be up there with the worst amp to recap?

Just finished the recap and have amp playing with an external turntable sounds good but not great so will be adding a few mods as recommended by Rob-NYC.
Man you guys that recap these for a living really earn your money I never want to see another one of these amps again.
All my rectified voltages from T101 are about 20 volts higher than the schematic shows will this be a problem??
My voltages are
Z 388 volts
V 336 volts
W 304 volts
Y 248 volts

Cheers
John

John,
Probably no big deal--see note #1 on the schematic--Also, what is the line voltage reading ?
Ron Rich

John, Ron pretty much answered it. This is an unregulated P.S. so higher-in =higher-out.

Also, as tubes age they draw less current and thus B+ load decreases causing a rise. This is primarily true with output tubes as they draw the most even at idle.

Rob

Thanks guys that's good news.
Ron line voltage is 120 volt
Rob all tubes are new.

Cheers
John

John,
I assume you read note 1 by now--and do not have the exact meter specified to have been used, to take those readings?
If, as I would guess, you are using a newer, "digital" meter, it won't read the same as the one specified--also, although I can't be sure,
I think that the phonograph voltage rating, at that time, was 110 ? All in All--I see no problem--
Ron Rich

Just getting back to testing this amp outside of the cabinet, amp doesn't seem as loud as I thought it would be???
Also the speaker test doesn't seem to work even though I have the 6v ac leaving the molex plug???
Any ideas

Some basic questions aside from the obvious ones about tube condition and B+ voltages:

Since you are outside the cabinet; what speakers are you using? How are they connected? What impedance -(supposed to be 16 Ohm).

What source are you using?

Have you removed the 6BJ6 AGC control tubes for this test?

Seeburg designed their amp gain structure so that when all was normal (with AGC) and good drivers & finals, it would not be possible to over-dirve the power stage. The speakers used were very efficient to make use of the limited power available. While those amps are rated @ around 22w/ch, the reality is about 10 rms/ch.

Rob

Hi John,
If you have a "Molex" brand plug--that's a BIG problem, since Molex was not in business when that phono was built !
They used "A-MP Loc" brand plugs in that model --
If you have 5-6 volts AC at the A-MP plug, but ho "hummmmmmm" from the speaker, there is definitely something wrong beyond the amp. Are you measuring with the plug on, or off ? It's possible that wire(s) are cut ( usually due to incorrect wire dressing, at/near the door hinge), the plug is not mating well, or a wire is poorly crimped, or the connectors on the crossover are not good, or--the speakers are "blown" ?? Ron Rich

Rob

Thanks for the reply I am using the existing 16ohm speakers but also trying tweeters to improve sound as per previous conversations with you.
Source is a Teac record player.
I have tried the machine without the AGC tubes with no noticeable difference.


Ron,

As the speakers are working I discounted bad connections, I will recheck.
Voltages were checked with the plug inserted.

Cheers

John