Univox u-1236 - oil coupling caps, previous owner's mod

[MonoPoly]

I bought a Univox U-1236 that wasn't passing any signal, but it turned out it just needed a new preamp tube to pass signal. After cleaning/retensioning the tube sockets and the pots, the scratchiness and, to my surprise, the 60/120hz hum largely disappeared. The amp now mostly sounds great. The only issue remaining is white noise that increases with the volume knob. The filter caps had been recently replaced with proper value Spragues. They test fine, as does the B+, 330v screen voltage, and all the resistors I tested. My next step is to check the atlas oil coupling caps for DC leakage, just waiting on my mouser shopping cart to fill up so I can have replacement film caps on hand while the pcb is removed. Would it make sense to just replace the 50 year old oil caps regardless?

Am I understanding the schematic (attached) correctly in that there is a gain stage after the phase inverter - is that weird?

It looks like there was a 1000 pf capacitor added from between the V3 cathode and bias resistor to between a load resistor and coupling cap of V3. What's that about?

[astronomicum]

Yes. It would be a good idea to replace the oil caps. Nearly all the caps I have tested from several Univox restorations were leaky. I have seen a similar cap added on two U235PB units (the 1235/6 little brother). Not sure of its purpose and will be interested to see what responses you get.

[sluckey]

It looks like there was a 1000 pf capacitor added from between the V3 cathode and bias resistor to between a load resistor and coupling cap of V3. What's that about?

It's connected between plate and cathode. Does it look like a factory job? Could be a band aid to remove some harshness or high freq. oscillation.

[astronomicum]

On the two U235PB units I noticed the addition of this cap, it looked to me like a factory job. In both cases, the mod included a second parallel 20uf cap on B3 supply with parts identicle in both cases.

[astronomicum]

While you are replacing the coupling caps, you would also want to increase the size of the V3 47k Plate Load resistors to at least 1 watt. The 1/2 watt resistors are undersized dissipating between 0.5 and 0.6 watts and are nearly all off spec. You may notice a darkening of the circuit board around the resistors indicating how hot the resistors are getting. Also check the V2 Plate Load resistor values as I have noticed that these are often times off spec as well. I am finishing up a U-1236 restoration and am doing the final testing. I have tested the unit with the 12AX7 in the V1 position and have measured a much improved overall gain profile and less noise when it is in that position. I was curious to see how the amp would perform because just prior to this restoration, I restored a later model U235PB that had the chassis stamped with a 12AX7 in the V1 position (all earlier models were all 12AU7s) and due to the nearly identicle architecture of the two models, I decided to test it out. It is also curious that it is not uncommon to find the U-1236 chassis stamped with the 12AX7 in the V1 position but paper labels over the chassis stamps switching the V1 and V2 valves to match the schematic. I am eager to understand why they made the switch.

[PRR]

> there is a gain stage after the phase inverter - is that weird?

If you live long enough you see many "weird" things.

This is a bastard between a Fender and a Williamson. We all know Fender's long-tails. The Williamson had low-Mu gain, cathodyne, and low-Mu push-pull driver like this. The final driver is not asked to also phase-split or accept NFB, it's just grid-grunt. I don't think it is techically needed. I do think the Elk/Univox designers were trying new things and avoiding the obvious (potentially patented).

What is more weird is two low-Mu stages under NFB for an input. The whole amp may be "too clean" for some players; OTOH many players will be fine.

[MonoPoly]

Thanks for the replies I'll have some spare load resistors on hand as well.

So far, I'm really only played bass through the Univox, so I like how clean it is. I don't really foresee using it as a guitar amp, since a deluxe reverb is pretty much more than enough for my guitar needs. Paired with a modern, ported bass cabinet, I really like the univox as a low volume bass rig. It might be due to the lighter weight or the modern cabinet, but I'd take that rig over an Ampeg B15.