Tweed Pro

[trem]

A friend of mine bought one of these amps.
It has Nine Pine sockets in the preamp.....but has Caps/Resistors for the Eight Pin Schematic
http://el34world.com/charts/Schemati...er_PRO_5C5.pdf
http://el34world.com/charts/Schemati..._5d5_schem.pdf

It was not working, had a broken fuse holder and some leaking Elytics.
I replaced the caps in the power supply, and installed a new fuse holder and a new power and stand by switch. The 250k plate resistors had drifted Way Up, so i replaced those. .
The amp now "Works".
The problem is that it seems to be wired for one schematic and have the parts for another schematic.
If i have it right, it has parts for a 5C5 but is wired for the Noval Sockets of the 5D5
There does not seem to be any Cathode resistors or caps. Cathode to ground measures 1.7 Ohms
I guess that is how they ran the 6SC7, but this amp is wired for the 12AY7.
Maybe i am missing something basic.?
How did this ever run 12AY7 with no Cathode Bias Resistors.?
How should i proceed with this.? I am trying to change as few parts as possible, but it already has new switches, fuse holder, plate resistor and Those coupling caps.
I guess what i am really asking is.....should this amp have Cathode Resistors.?
Thanks For The Help

https://i.imgur.com/kxF5EZq.jpg

https://i.imgur.com/X0NfSC6.jpg

https://i.imgur.com/vuRM0ly.jpg?1

[mozz]

Cathodes going to ground would be grid leak bias. Some early tweeds used that, with 6SC7, 6SJ7, others. I do recall seeing a 9 pin with grid leak bias but not sure if Fender did that or not. Often the paper attached to the inside cabinet did not always follow the actual circuit. Do the sockets look original?

[trem]

Yeah Sorry......my best bet would be i am the first person to touch this amp since it was born. All the solder joints looked original. I would say the nine pin sockets have been there forever.
Grid Leak Bias...i will have to Look at Merlin's Book, but is that why they are using 5 Meg resistors in this amp.?
Thank You

[pdf64]

Yes, together with dc blocking caps at the inputs

[trem]

Yes, together with dc blocking caps at the inputs

Ah Yes.....Right.
So what do i do here...if this was your amp, would you leave it the way Fender set it up.?
I guess this was a bit of a "Mistake".....but it must have run this way for many years.
Would you guys leave the bias the way it is.?
Would it be a "better" amp if it were to run the 12AY7 with a more traditional Cathode Bias scheme.?
Just wondering what i should recommend to my friend.
Thanks Again

[Enzo]

Make it work. As far as I can read here, the only problem it has is it doesn't match the schematics you are looking at. It has worked for 50 years, so why change it now?

It isn't a mistake, it is just a change.

You tell me what "better" means. If it has leaky caps, or drifted resistors, change them. If it has noisy controls or jacks, clean them. But unless there is a compelling reason to rewire it into something other than what it is, I would leave it this way. Do you want to say to your customer, "Here is your amp, it used to be an unusual model, but I changed it into one like a million others."?

[pdf64]

Probably best to go one way or the other; a halfway house that allows a ‘no bias’ condition seems only to have benefit as being a museum piece quirk.
The lack of blocking caps and cathode bias probably makes instrument volume controls scratchy?
Perhaps use whichever bias method works best with the existing layout.

[Enzo]

Grid leak bias is not "no bias" it is just bias not derived from cathode current.

[pdf64]

If there are input blocking caps (?) then yes, no need to mess with it.

[J M Fahey]

PLEASE look at the Fender Original 5C5 schematic, we canīt argue what *they* wrote, can we?

It clearly shows that 5C5 can be built with Octals (6SC7) *or* miniatures/novals 12A*7 , obviously what many call " a transition model"

So what you have follows one official version, nothing to be surprised at.

To boot, schematic includes a Factory NOTE , stating latest models in that series use miniatures instead of octals.

As they say: "straight from the mouth of the horse"

[Enzo]

The layout drawing also notes that later versions have added a 1 meg feedback resistor from the speaker out.

[trem]

The layout drawing also notes that later versions have added a 1 meg feedback resistor from the speaker out.

THAT is something i did not notice. I will have to look at the chassis and see if this 1M is there.
Anyway......i guess the best way to go is leave it the way it is (for now) and try it with a guitar. If it sounds good, i will just leave it the way Fender made it.
Thanks

[Enzo]

Just a general suggestion, many schematics have notes, and it always pays to read them. Some are ordinary things like "all capacitors are in uf unless noted" But some are more specific like the ones we mentioned. Sometimes adjustment values are noted. Some even have serial numbers as in above this number such and such change was made. Sometimes you find the schematic just has part numbers like R4 and C5, but the values are in the notes.

[trem]

Hey Enzo -
Yeah...words of wisdom.
I made the mistake of thinking just that.......that the notes were about resistors being 1/2 watt etc etc.
It has been a while since i worked on an amp. I have a lot of "stuff" to relearn.
Thanks Again