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Questions about differences between AA964 And AA270 Vibrolux Reverb

Figaro

Silver Supporting Member
Messages
13,130
A couple of questions about the differences between the blackface AA964 and silverface AA270 Vibrolux Reverb circuits...

What is the purpose of the 5/25V capacitor and the two 150 ohm resistors on the AA270 power tubes? Should they be removed?

What difference did changing to the AA270's 4.7K resistor near the filter caps from the AA964's 10K resistor make? Was this done because of the AA270 power transformer's higher voltage?

Thanks!


 
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Bob Arbogast

Gold Supporting Member
Messages
1,070
I think you're referring to the AB568 schematic. Those extra resistors and the 5/25 cap look like a semi-cathode-bias setup. Others will be able to explain it more.

The 4.7k vs. 10k provides higher DC voltages for the phase inverter, preamp, and reverb return/recovery stages to work with.

 

Figaro

Silver Supporting Member
Messages
13,130
I think you're referring to the AB568 schematic. Those extra resistors and the 5/25 cap look like a semi-cathode-bias setup. Others will be able to explain it more.

The 4.7k vs. 10k provides higher DC voltages for the phase inverter, preamp, and reverb return/recovery stages to work with.
Thanks! What differences do the higher voltages make?
 

swiveltung

Senior Member
Messages
14,485
I think the caps and resistors on the power tube sockets are simply to quiet the amp some. Just remove them or leave them. There are good and bad opinions on them. Not sure why they changed the voltages, but did that on a few different amps IIRC. Likely due to going to the 5U4GB rectifier I suppose. Some parts of the PI circuit were changed also.
 

Muttlyboy

Gold Supporting Member
Messages
2,511
Just a hobbyist here, but I'm under the impression that 4.7k resistor vs the 10k resistor as well as the 47k plate resistors on the phase inverter tube was to increase the clean headroom
 

Muttlyboy

Gold Supporting Member
Messages
2,511
I think the caps and resistors on the power tube sockets are simply to quiet the amp some. Just remove them or leave them. There are good and bad opinions on them. Not sure why they changed the voltages, but did that on a few different amps IIRC. Likely due to going to the 5U4GB rectifier I suppose. Some parts of the PI circuit were changed also.
That's a good point,
I didn't even think about the 5u4 rectifier tube.
I just assumed that the 5u4 was to accommodate for the higher wall voltage.
 

hunter

Silver Supporting Member
Messages
7,912
The different resistor values in the "ladder" affect the plate voltage on the preamp and inverter tubes by changing the way voltage is split off among the tubes. In general, the 4.7 k drops less voltage leaving more voltage on the ladder for tubes and so the plate voltages are higher on the SF preamp and inverter tubes, which, in general, gives more headroom and cleaner operation. The combined bias balance and cathode bias (which is where the cathode caps and resistors appear) circuit was only in play for a short time ca 68. This was one of the most scorned BF to SF changes and was quickly abandoned by Fender. I wouldn't expect to encounter that on an amp made much beyond 70. If you play quiet it won't matter that much but in general, I take that cathode bias stuff out. They do not appear on the AA270 schematic FWIW.

You also have those 2000pf grid suppressor caps on the later Fenders. As noted they are there to help tame parasitic oscillation. Some say they rob highs. Not that much difference to my ear, but if your amp runs OK without them, removal is OK. I take em off.

hunter
 

Figaro

Silver Supporting Member
Messages
13,130
The different resistor values in the "ladder" affect the plate voltage on the preamp and inverter tubes by changing the way voltage is split off among the tubes. In general, the 4.7 k drops less voltage leaving more voltage on the ladder for tubes and so the plate voltages are higher on the SF preamp and inverter tubes, which, in general, gives more headroom and cleaner operation. The combined bias balance and cathode bias (which is where the cathode caps and resistors appear) circuit was only in play for a short time ca 68. This was one of the most scorned BF to SF changes and was quickly abandoned by Fender. I wouldn't expect to encounter that on an amp made much beyond 70. If you play quiet it won't matter that much but in general, I take that cathode bias stuff out. They do not appear on the AA270 schematic FWIW.

You also have those 2000pf grid suppressor caps on the later Fenders. As noted they are there to help tame parasitic oscillation. Some say they rob highs. Not that much difference to my ear, but if your amp runs OK without them, removal is OK. I take em off.

hunter
Thank you, sorry. You're correct, only the AA568 has the the cathode caps on the power tubes.

I'm trying to decide to do a partial or complete blackface conversion on an AA270 Vibrolux Reverb. I'm going to convert the bias balance to blackface adjustment.

Do you think it's better to have the higher silverface cleaner preamp voltage or the lower blackface preamp voltage? I guess it's easy enough to change that one resistor to hear the difference for myself.
 
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hunter

Silver Supporting Member
Messages
7,912
That resistor is in the belly pan and not too hard to swap out. Better or not depends on you and how you use the amp. I have left em alone and I have changed them too. Probably worth trying both ways. I believe you'll need to get pretty loud to get much sense for the difference.

hunter
 

Figaro

Silver Supporting Member
Messages
13,130
I changed the .01 coupling cap going into the phase inverter to the .001 blackface value. This made it sound much better, clearer without all the low mids that the .01 let through.
 

neteraser

Member
Messages
1,994
Yep, Leo got it totally right, everything else is a get away. But quite an accurate one. If you don't mind, I'd advice to love it as is. These Silverface amps are well-thought out standalone concepts. I'm afraid it's easy to get to the point where you have no blackface amp and no silverface one, but rather something with unproven or possibly incorrect concept. But I don't know, it's up to you.
 

neteraser

Member
Messages
1,994
Thanks man!

I agree with neteraser. They don't really end up sounding better or like a blackface to me.
Possibly because transformers also got tweaked, and of course the rectifier. I'm not a big expert on Silverface amps, but I'd be very careful myself. And here is why.

Of course like many tube amps enthusiasts, I also had to check what 2000pF caps are all about, as well as 330k in the PI and some power supply changes, and I think it all makes sense together. Just how did they guess to put those 2nF caps there? I currently believe it influences some virtual impedance of the power stage, allowing (or maybe even asking) a slightly tweaked OT for more of that 'Silverface' tone. Take them out, and you'll probably be missing something as far as a Silverface amp is concerned. Or I guess so. I have no real clue actually and I'm not afraid to say so.

Actually I've been working on my take on the BF Deluxe, and for some period, the power tube (I used a different power tube to get about 2 watts) just wasn't happy without 2nF caps (I didn't have the right value, so I used either 1nF or 2nF). That pissed me off because I couldn't get why is that back then, and also I liked blackface tone better, but unfortunately the tube said 'no' leaving me wondering. For a long time I didn't pay attention to this: it works, means it's good and you don't wanna touch it (by a programmer's law). Then I occasionally found a right transformer partner, he told me what he thinks is the right primary impedance on the OT for that tube / with those voltages / in that circuit, which was a little surprising because it was higher than what you'd usually figure out and first I couldn't believe it and thought it's not correct. But finally I get it, it works in it's blackface form nicely. Tell you, there were moments in my life when I just sat on my ass and thought "what a fool I am, I had NO clue at all (of how it was really done)".

I think blackface era amps are very wise in design and resulting sound. And Silverface amps are also a work of art. It's hard to predict if blackfacing will work or ruin the amp. But I told you a few considerations from my experience. Everything is not so simple about tube amps. Could be irrelevant to your particular amp. The point is just: be accurate or probably better don't do it. I remember the time when tweaking amps was fun to me. Now I can say I'd rather stay away from this job, but I gotta finish what's started, and if my interpretation of Fender will make someone in Russia a little happier, I'd be equally glad. That said, it sounds not worse than a Frontman. Yeah, definitely not worse than a *ing Frontman. (the joke is that Fender is on a different level anyway. Leo worked 17 years having most of what he needed.).

WARNING: this post is full of vanity.
But hope a wise man would get the idea. There was probably a better way of putting what I had to say about blackfacing. I'm going to regret it.
 
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bigben55

Member
Messages
2,895
Looks like the VR didn't get changed "too" much between the BF and SF years, compared to some of the other amps.
 

Figaro

Silver Supporting Member
Messages
13,130
Looks like the VR didn't get changed "too" much between the BF and SF years, compared to some of the other amps.
The changes CBS made to this 76 silverface VR were minimal. The biggest single sound improvement was changing the silverface .01 phase inverter coupling cap to the blackface .001. I also removed the 2000pf caps on the power tubes and made the bias adjustable. It sounds awesome.
 






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