SHFA4 Amp questions

History:
I got the AY160 in July 2013. The SHFA4 amp didn't sound "right" and the tone switches did nothing. I rebuilt the amp with -all- new electrolytic caps (Including PS Caps) and replaced any out of tolerance resistors. I also replaced all 4 power supply resistors. I over-wattaged the PS resistors a bit, for example the 100Ω 7w was replaced by a 100Ω 10w.

The amp sounded great after the rebuild and the tone switches now work. It otherwise perfect except for a slight (but annoying) power supply hum.

One channel began to crackle after a few months. It sounded like an approaching thunderstorm on an AM radio.

I isolated it to this (new) .047/630 coupling capacitor:

The plate voltage is what I would expect it to be and I replaced the defective coupling capacitor with the same brand and value:


The amp sounded clean for a few months, then recently began the same crackling. I have it on my bench now and isolated it to the very same coupling capacitor.

Question.
Do you folks run into brands which are not reliable? What would you suggest? I hate to put the same kind back in.

About the PS hum. I did some measurements, and I believe I will need to change cap values of C120C and C120D to smooth this out. They are both rated 47/450, very close to the original can. C120 A&B are 22/450.
Red numbers are measured DC voltages. What do you suggest:



Last question, it's about output tube bias. Is this something I should check or assume it is alright?
Below is the schematic of a channel. I notice the cathodes are tied directly to ground (I am used to seeing a resistor-cap circuit there). I would normally take a voltage drop measurement across such resistor/plate voltage to obtain current.
In this case, it seems I'd need break the connection on pin 9, and place an ammeter in series between pin 9 and the transformer:

→ Is it worth checking bias or is it a big waste of effort on this amp?

I plan to do some input/output waveform tests and I goal is to get this amp sounding best as possible.

Any suggestions are very much appreciated.
-Steve

Hi Steve,
Most--if not all, of the coupling caps I see, have no "brand name" on them ? I don't think I have run into the one shown, and certainly would replace it with another "brand", if It failed twice, for me---
As for the 7 watt resistor--Seeburg saved $$ by not going to a 10watt unit--but I can only recall seeing one "bad" 7 watt unit there--don't remember what was wrong--think a lead broke off ?
Hummmmmm--Well,this is a deep subject--since I can't hear it, I'm wondering if you might be "obsessing"a little--as all these amps have a slight power on, hum.( Which I was tolt, by an old time "amp man", is the "warm tube sound" ?) Ron Rich

It is unusual for the same component in the same position to fail repeatedly if it operated within spec's. The noise you describe is dielectric failure in the cap. This means DC from previous stage is beginning to leak through and you should be able to easily measure this with a meter.

There exists one other possibility; the plate resistor of that stage. Sometimes when a resistor (or diode and transistor) is failing due to poor internal connections, heat from a soldering iron will temporarily fix it. Again, simple DC measurements on the low side of the coupling cap will confirm/deny this.

FWIW: I use an old stock of brown Elmenco or Sprague orange caps that I bought in 1988 from a company going out of business here in NYC. I test each one first and have never had an in-service failure.

HUM: As Ron points out this sort of amp does have some intrinsic hum. Primarily due to the AC on the tube heaters and made worse when there is heater-cathode leakage. This is why professional equipment uses filtered DC to heat high-gain stages. However, it should not be noticeable when you are more than a few feet from the speakers. Is the hum 60Hz or 120Hz? If 60Hz adding more filtration won't help in the high gain stages. It -might- help in the fixed bias supply, but that one is pretty-well filtered already. The other obvious questions are: Is it ahead of the volume control, or after? Does removing the pickup plug change things?

Checking bias: In a fixed-bias circuit you simply measure at the last filter and again at the tube grids. Schematic calls for a minus 35 at the filter so allowing for slight loss across the two resistors at the grids and meter loading, expect around minus 33 VDC at the grids. FWIW I always replace the original selenium rect with a new silicon diode. This results in around -36VDC at the grids. You will see -slight- variation at the different grids. But if more than a few volts, pull the tube and check the resistor. If resistor Ok, connect meter to the grid terminal, put tube back and watch if the voltage drops as the tube warms. If a tube bias changes a lot in fixed-bias circuit, you have a very gassy tube.

Rob/NYC

Thank you so much for the responses.

The only time I really notice the PS hum is when I'm near to the speaker, such as cleaning the bottom chrome rail or the grille crowns. Also other times like in a quiet room standing in front of the jukebox making selections. The hum occurs when ever "B" voltage is present in the amp, such as when it's muted during carriage scan. You can hear when the 5U4 begins to warm and operate after making a first selection. The volume position has no effect.
Maybe Ron is right, that I am just over thinking this. It's just that I didn't notice it before re-capping the amp. I can't say for sure whether it was there or not. I can live with it.

I'll see if I can find the right coupling capacitor values in Tantalum, and replace both channel A and B for all things equal. Also replace the plate resistors. This is also a good opportunity for me to re-cap my SS speed control.

The only silicon diode I know of is in the Tormat selector unit TSU4 (CR501). This selector unit has also been recapped and working properly. Should this silicon unit automatically be replaced with a new bridge diode? If so then most any 5a 1kv package will do. Right?

Steve,
Use a 1N4002, or better--Ron Rich

Also, I am interested in running some waveform performance tests on the amp. This input interface is posted on the schematic:



I was wondering if anyone knows the choke and resistor values within this PU-249755 interface. I haven't been able to find any information on that interface at all.

It is easy enough to build the voltage divider and feed it into both channels... I was wondering about this PU unit.

Steve,
That's the "cartridge" Seeburg used in your model phono --(P{ick} U{p} # 249755)
Ron Rich

SteveFury wrote:Thank you so much for the responses.

The only time I really notice the PS hum is when I'm near to the speaker, such as cleaning the bottom chrome rail or the grille crowns. Also other times like in a quiet room standing in front of the jukebox making selections. The hum occurs when ever "B" voltage is present in the amp, such as when it's muted during carriage scan. You can hear when the 5U4 begins to warm and operate after making a first selection. The volume position has no effect.
Maybe Ron is right, that I am just over thinking this. It's just that I didn't notice it before re-capping the amp. I can't say for sure whether it was there or not. I can live with it.

I'll see if I can find the right coupling capacitor values in Tantalum, and replace both channel A and B for all things equal. Also replace the plate resistors. This is also a good opportunity for me to re-cap my SS speed control.

The only silicon diode I know of is in the Tormat selector unit TSU4 (CR501). This selector unit has also been recapped and working properly. Should this silicon unit automatically be replaced with a new bridge diode? If so then most any 5a 1kv package will do. Right?

Steve, no, if the hum is that noticeable, it indicates a minor fault. One thing to check is to determine if this hum is either both channels, or just one. It is 60Hz or 120Hz? If 60 Hz it is probably in one of the 7199 tubes. There has been an issue with heater-cathode leakage in some of the recent manufacture 7199 tubes causing hum. In theory, a pair of balanced 6973 tubes will cancel this sort of hum if it is created in the splitter (triode) section of the 7199 because it will appear at each tube phase-opposite.

You should also check the B+ for any out of tolerance readings in the P.A. section. this would indicate either inadequate filtering or heavy drain due to gassy tubes or inadequate bias. This will be 120Hz.

As for checking frequency response, be prepared to NOT see typical RIAA from the preamp. Seeburg used only partial eq. there and major boost in the contour taps on the volume control. This gives bass boost at low level and bass reduction as the pot is turned up taking some load off the speakers. I've always bypassed those early pre's and used an external one with slightly modded high-frequency response.

Rob

Guy's,
I still think that's pretty normal for that amp-if you have to "get down" to hear it-'course, I am not listening to yours ??
Ron Rich