Replacing Silicon Diodes with F.R.E.D.s

[TD_Madden]

After reading about an AC50 retrofit in the Sep '06 issue of ToneQuest, I'd like to replace the four rectifier diodes in my newly-acquired '66 AC50.

Anybody know what type to use and where I might find them?

[Bruce / Mission Amps]

After reading about an AC50 retrofit in the Sep '06 issue of ToneQuest, I'd like to replace the four rectifier diodes in my newly-acquired '66 AC50.

Anybody know what type to use and where I might find them?

More Net driven hype.
Don't waste your time.... I doubt they are bad now but a foursome of 1N5408s is all you need to replace those.
Spend some money on new filter caps, replace the cathode bypass caps and fiddle around with the bias supply so you can get the two EL34 power tubes idling around 17-18 watts for a great tone.

[TD_Madden]

Bruce,

I 've heard you are the go-to guy on these amps out West.

A question about biasing this thing...supposedly getting a 2.2v reading across the 47-ohm resistors on pins 1/8 biases them properly, but how do I find out how to check ma on this thing? 2.2v means nothing to me.

Also, I thought this thing was supposed to be a 45-50 watt amplifier......

Thanks in advance.

[TD_Madden]

nevermind.... current= volts/ohms, so that 2.2v/47=47ma

I notice the '76 schematic says 1.8vdc....maybe a safer choice.

[TD_Madden]

As I decrease the voltage...hum increases...is that normal for this amp?

[bob p]

After reading about an AC50 retrofit in the Sep '06 issue of ToneQuest, I'd like to replace the four rectifier diodes in my newly-acquired '66 AC50.

Anybody know what type to use and where I might find them?

what's the objective in doing this? i'm not familiar with the article you've mentioned, but there are all sorts of opinions floating around. some better than others.

if diode switching noise is a concern for you, instead of buying a bunch of FREDs, it might be worth considering just putting a snubber circuit across your diodes.

http://www.hagtech.com/pdf/snubber.pdf

R.G. was kind enough to provide that reference in another thread:

http://music-electronics-forum.com/s...ight=rectifier

[Bruce / Mission Amps]

As I decrease the voltage...hum increases...is that normal for this amp?

No not really. Do you have around -50v to -55v on the EL34 grids?
I've refurbished a bunch of these amps and have always found that using a pair of 1% to 5%, 10 ohm 1 watt resistors, one on each cathode, was much easier to deal with.
Those bias supplies (two pots for level and balance) are tricky and I like to rebuild the bias supply section as soon as possible.
First thing to do is replace the bias supply filter caps with 10uf/350v ones.
Look carefully at the schem... it uses a lot of caps.

Yes, 47ma at 470vdc is around 22 watts each. Too high for a great sound and of course they are not Class A amps.
But, if the main filter caps are still good, the B+ at the choke and first filter cap will be closer to 490vdc to 500vdc.
You probably will find the filter caps need replacing if they are the old ones.
Anyhow, get the power tubes to around 16-18 watts per tube to get a great Voxy tone.
Also, they are very bright amps if plugged into the brilliant channel.
I've seen a ton of those amps but have never seen one with the brite switch that is on the schematic. Does yours have it or is the 150pF cap soldered on the brilliant channel volume pot?

[TD_Madden]

it's worse than that...at the suggested setting (2.2vdc) it was 47ma at 500v: 23 watts...on a NOS tube that shouldn't be biased over 15 watts (per Mike Kropotkin).

All the electrolytics have been replaced in this amp....but I'll doublecheck the bias caps.

By the way, the ToneQuest article I was referring to was in print originally in Sep '06.

Here it is: http://www.tonequest.com/pdf_pubs/sa...ep06_proof.pdf

My ignorance on this circuit is pretty obvious...any assistance gladly accepted!

[TD_Madden]

Bruce,

I've used the 1-ohm cathode-resistors for awhile to bias other amps.....but this thing has 47-ohm resistors on those pins. So you mean that I should replace them or add the 1-ohms in series? whew, I'm confused....no doubt.

Also, it appears that the stock biasing caps are 8mf/50v......these should be replaced with 350v versions? How much voltage is in that area?

[TD_Madden]

Whew...egg on face again.

instead of the fuzzy schematic I looked at the circuitry. The bias caps are 8mf/350v. Any reason to bump these up to 10mf?

Got the Teslas in and bias at just over 30mA each....between 15 and 16 watts. Also found a loose solder blob hanging between pins 8 and 7 on one of the outputs......

Now to figure out how to remove the residual hum. Even with all 9-pin tubes pulled, it has the hum.

[gdgross]

UF4007s are just as cheap as 1N4007s. And they supposedly have a faster switch off time.