Supro 1688TN Power Section

[Vez]

Hi, forgive me if I posted in to wrong section.
I just bought a Supro 1688TN. The fi!tering in the power section seems weak.
The order is 20uf @450vdc, 10@450//20@450, 10@450
Are the voltage ratings added together when in parallel?
The tube lineup in this amp is 5u4, 2 x 6L6gc, 4 x 12ax7.
It seems to me this need more filtering and a rating at 450 is inferior.
Am I wrong?
What should I increase and how much?
Also, this is cathode biased.
Before any work, I brought the amp up to 115 on the variac and it hummed. Not badly but a 120hz hum was noticable. I traced the power section and attached it here.
Btw this is one of those dual chassis amps (like some Magnatones). The power section is a single chassis at the bottom of the cabinet and preamp section is on top. Connected with an umbilical conduit.
Thanks Rick

[sluckey]

Those cap values and voltage ratings are very typical for that time period. They are not under sized.

[Vez]

Those cap values and voltage ratings are very typical for that time period. They are not under sized.

I thought the first capacitor is supposed to be the largest. Is it not cal!ed the reservoir capacitor? Also typical supplies were 110 to 115Vac. Now it's 120 to 125vac.

[Vez]

Is this forum just for Hoffman amps?

Sorry, dumb question..."Hoffman Amplifiers Forum".

[sluckey]

Is this forum just for Hoffman amps?

No. All tube amps are welcome. Doesn't even have to be a guitar amp.

[PRR]

Capacitors used to be relatively expensive. 8uFd was often used at the first cap, then 16uFd. Push-pull stages are relatively insensitive to ripple.

Supro was not a high-price brand. "Supro was basically the budget brand of companies such as National." They often cut close to the bone. Hey, every town had a radio repair shop.

Yes, look for unexpected high voltage on old amps on new wall outlets.

[tubenit]

Leon C is very knowledgeable about these amps and is an amazing player!   


This has remained close to my favorite amp demo of all!  If you can get yours sounding like this ........................... 


Supro 1688TN and Wet Rig Demo by Leon C - YouTube

[Vez]

Capacitors used to be relatively expensive. 8uFd was often used at the first cap, then 16uFd. Push-pull stages are relatively insensitive to ripple.

Thanks for replying. Do you mean the 120 hum from old caps originates in the preamp? Very interesting, I didn't know that.
What was their reasoning to put the 20uf parallel with the 10uf?  For better filtering? Higher voltage rating? Are these two holding the bulk of the energy for the amp? So the "reservoir" Doesn't have to be the first cap?
Thanks again, Rick.

[Vez]

Leon C is very knowledgeable about these amps and is an amazing player!   


This has remained close to my favorite amp demo of all!  If you can get yours sounding like this ........................... 


Supro 1688TN and Wet Rig Demo by Leon C - YouTube

Thank you Tubenit. I will look him up. I assume he's on this forum? He really got that amp sounding great!  Ill ask him what he did to his.
I just bought mine. I was lucky,  the original Jensen's are in good shape.

[sluckey]

Thanks for replying. Do you mean the 120 hum from old caps originates in the preamp? Very interesting, I didn't know that.

That's not what he means at all. He simply said the push/pull stage is relatively insensitive to ripple. This is due to the common mode rejection characteristic of the push/pull stage. Even though the ripple hum is very high (sometimes 8-10V) at the first filter cap node it doesn't matter because it will be cancelled out due to the push/pull action. This is not true for a single ended power amp.

What was their reasoning to put the 20uf parallel with the 10uf?  For better filtering? Higher voltage rating? Are these two holding the bulk of the energy for the amp?

I think that axial 20µF was added later on by a service tech to reduce hum due to a failing 10µF inside the can. It was very common practice to simply add another cap in parallel to the single failed can cap section in order to avoid the expense of replacing the entire can. BTW, putting caps in parallel does not increase the voltage rating. The voltage rating is still the same as stamped on each cap.

My Supro S6424 uses that same 20/10/10 cap can and there is no additional axial cap.

So the "reservoir" Doesn't have to be the first cap?

All filter caps act as a reservoir. It became customary to refer to the first cap after the rectifier as the 'reservoir' just because it has the highest current drain due to supplying the power tubes (I guess). I don't know who was the first to use the word 'reservoir' but it caught on and stuck.

If I was replacing the filter caps in that amp I would just use three axials, 20µF, 10µF, and 10µF and throw away the extra 20µF axial cap. 20/10/10 filtering is fine for that amp.

[PRR]

Do you mean the 120 hum from old caps originates in the preamp?...

120Hz buzz comes from using 60Hz power from the walls. The full-wave rectifier makes it 120Hz at hundreds of volts. The reservoir cap reduces that to dozens of volts 120Hz. The push-pull stage mostly cancels its supply crap; anyway large-signal stages overwhelm buzz. The multi-stage R-C reduces that dozens-Volt 120Hz for the more sensitive preamp stages, which also lack push-pull rejection.

[Vez]

I think that axial 20µF was added later on by a service tech to reduce hum due to a failing 10µF inside the can.

Thanks for replying Sluckey but that's not likely. I too have seen components tacked on at later dates but, the 20uf in question is dated "235-6151" and the solder joints look consistent and uninterrupted. This date aligns itself with the rest of the components in the amp. A similar Valco made amp from the same era, the Gretsch 6159 with the same tube line up has the exact same filters as this amp.

My Supro S6424 uses that same 20/10/10 cap can and there is no additional axial cap.

The 6424 is a completely different amp with different tubes.

[bmccowan]

Thanks for replying Sluckey but that's not likely. I too have seen components tacked on at later dates but, the 20uf in question is dated "235-6151" and the solder joints look consistent and uninterrupted. This date aligns itself with the rest of the components in the amp. A similar Valco made amp from the same era, the Gretsch 6159 with the same tube line up has the exact same filters as this amp.

I concur - Valco did this a lot. I imagine it was a parts bin decision as I cannot think of an advantage. BTW these amps sound great! I would encourage keeping all stock values. I also would discourage replacing the ceramic disc capacitors with other - they are part of the sound. Second the advice on Leon C. Check out his videos - wish I could play like him.