Forte Audio 1A problems... PLEASE HELP!!

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Does anyone happen to know the value of the bias resistors in the Forte 1a? Mine have gotten pretty ragged and I have to keep twisting the wiper on one of the channels to get it to bias up again. I want to get replacements, I suppose I could pull one and measure it but if anyone happens to know the value I'd appreciate it. Thanks!
 
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Does anyone happen to know the value of the bias resistors in the Forte 1a?

If you mean the emitter resistors in the output stage, they are all marked 0.47 ohms. If you mean the resistors in the Vbe multiplier circuit, they are identified in the manual as R11 (1K), R12 (6.8K), R14 (1K), R15 (5K pot), and RT1 (1K thermistor). As long as you are in the mood, I would definitely replace the pot.
 
Thanks so much for your fast responses, Zen Mod and Joe Berry! In fact, there is writing on the pots but I wasn't able to decipher it as anything to indicate value, and Google search turns up nothing that I could find helpful. I'm sure it tells the info somehow, but heck if I can figure it out.

In any case, it appears the answer is that it is a 5K pot, thank you Joe Berry! I will replace them.

I bought this amp new in the early 90's, have listened to it virtually every day since. I'm continually delighted, even near 30 years on, by the sound of music through it. How many things that you buy can you say something like that about?

Thank you again for your help!
 
So, with the information I found here, I fixed my cold Model 3 problem. I checked the voltage across the resistors to start... 5 millivolts on one side, 2 on the other. I'm pretty sure these are just dirty pots that needed to be moved a touch. Adjusted them up to 25 millivolts. After 1 hour, I see about 40C on the fins. I put it back into my system and it sounds much better. The pots are super touchy. It's quite hard to get the 2 sides the same. I will try this for a bit... but I think I'm already feeling the need to try it a bit hotter. AND, what a cool thing to get a note from Mr. Pass. It adds some fun and comfort to us newbies! I'd love to buy a second model 3 if anyone has one for sale.
 
Model 1 and Model 3 power transformer

Hi, will be trying to mod a model 3 in a model 1 for 50 watts class A.
As I understand it and Mr. Nelson Pass suggested to have the primary of the power transformer wired for 240v, and once connected to 120v it will drop secondary volts to +/-42vdc so half a standard Model 3.
Then increase the bias to have a Class A operation.And of course with a good maintenance before and changing bias pots....
All of this is all Forte Model 1 and Model 3 have dual primaries transformers???
Thanks
 
oh my mistake .....sorry.:eek:

Well from my passed reading here on the bias issues model 3 converting to Class A, as I recall now Model 3 is +/-72Vdc then half should be +/-36vdc.

But is all Forte model 3's where built with a dual primary transformer??

Will have a Model 3 on my bench next week to confirm...hope I can mod it to a model 1 in Class A.....

Again kind regards Mr. Pass I think this forum is highly privileged to have you on line!
 
Capacitors and power transformer in Forté model 3

Had the time to open check power caps they are 24000uF/45v Mepco/Centralab 85C.
They all test equal or higher of 24000uF and ESR measured below 0,02ohms
quite good!
Transformer is multi-voltages primary 120V-240V and size is near 450-500VA
So B+ voltages are near +/- 35Vdc as I did not measured it yet.

Will replace the 5Kohms with multiturn trimpot for bias on both channels.
Re-grease thermal compound output transistors and recheck all connections and heat dissipated measured on series resistors and heat temp. with a laser sensor on heatsinks.
 

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Just bought a Forte Model 2 preamp as well. Just started looking for upgrades. The 2 47uf caps and the opamps look like potential targets to me. I have read what others are saying but can't see a conclusion on the best opamp replacement. Can anyone help with this?

Thanks in advance!
 
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