Blackfaced 68 super reverb with JBL's

[redshark]

I have a friend's super reverb here and he complaints about early breakup.
I already checked power tubes bias, plate voltage is @ 463 which is ok for this amp and bias was 32mA. but I adjusted to 43mA which is more appropiate with this plate voltage.
I noticed this amp has been worked on but the last tech decided to use ceramic resistors in the preamp, yes, not even metal films, plain ceramics so that bothers me, Does it contributes to the amp's harshness?
V1 and V2 are electroharmonix AX7's which I don't know how good or bad they are.
The amp with a Les Paul starts to break up at 3 in volume. Amp is dark in sound, I have to put bright switch on?

Any opinions? is this normal on this amps? I don't have experience with them. I appreciate opinions!! Thanks!!

[Reeltarded]

First remove those tubes. Almost anything is smoother. That might make all of the difference. I find them VERY much as the owner is describing the sound of his amp. Early and poor. Not bad for a PI somewhere.

[Colossal]

First remove those tubes. Almost anything is smoother. That might make all of the difference. I find them VERY much as the owner is describing the sound of his amp. Early and poor. Not bad for a PI somewhere.

I completely agree. There were three EH 12AX7s supplied with a DLite kit I assembled for a fellow and found them harsh. I couldn't get them to work in that amp anywhere.

Also, to delay breakup, you could change the 220k grid leak resistors in the power amp to 150k. Increasing the bias voltage to 42-44mA will certainly sound better, less cold and sterile.

[redshark]

Thanks!! I will experiment with preamp tubes other than the EH!!

I just can't stand the 3 100k resistors in both preamps (normal and vibrato) being metal oxyde!! Also I think the tech didn't have a 0.047 cap and he put 2 IC's in parallel. I noticed all the blue molded caps are gone!! all of them!! and replaced with polyester 715p which are good caps but all the blue molded gone looks to me like a ripoff!!

[Colossal]

715P are polypropylene caps.

Interestingly too, I just serviced a Deluxe Reverb with a JBL E120-8R. The owner complained of excessive high end. I tested the amp with an external 1x12 cab loaded with an EVM12L and it sounded far better than the JBL. I also found the DR combo cab to sound anemic and lacking low end. The owner bought an EV after I loaned him my 1x12 with the EV.

[Richie]

Blackfaced 68, meaning someone changed things? Things as simple as changing the volume pot. the taper could be off or bad pot. But a lot of times people say blackfaced. Or you get the ones where the person bought a lot of "new' parts,and changed them out, never measured etc.
And you see things changed, and some things left in that should have been removed first. So it depends on what was changed, and if the parts are correct,or old ones that have drifted etc. Are both channels the same far as coming on quick? can you post some pics?

[Richie]

Thanks!! I will experiment with preamp tubes other than the EH!!

I just can't stand the 3 100k resistors in both preamps (normal and vibrato) being cement!! Also I think the tech didn't have a 0.047 cap and he put 2 IC's in parallel. I noticed all the blue molded caps are gone!! all of them!! and replaced with polyester 715p which are good caps but all the blue molded gone looks to me like a ripoff!!

HA! 715 are polypropolene. At a low volume, it will probably be real super clean, but any distortion will sound harsh or unpleasant to the ear breakup. At this point, you might have to ask the owner what he wants or what he wants it to sound like etc. Seems everyone has a formula for fender amps.

[redshark]

Yes, someone changed LOTS of things but I don't think it was blackfaced cause the amp was stock AB763 so I don't think it needed to be blackfaced except for the cosmetics (faceplate).
Thanks for letting me know this caps are polypropylene!! I didn't know it!! LOL I guess I spent too much time on the trainwreck side!!
The resistors I talk about are metal oxyde the triplet of 100k in the preamps of both channels! I will change them ASAP, replace the tubes and see how it works!!

Thanks for the suggestions guys!!
Ritchie...the owner wants it to sound like a typical blackface super reverb
no more, no less!!

[redshark]

715P are polypropylene caps.

Interestingly too, I just serviced a Deluxe Reverb with a JBL E120-8R. The owner complained of excessive high end. I tested the amp with an external 1x12 cab loaded with an EVM12L and it sounded far better than the JBL. I also found the DR combo cab to sound anemic and lacking low end. The owner bought an EV after I loaned him my 1x12 with the EV.

Yes, that is my experience with deluxe reverbs. I had serviced 3 of them and being a rocket player (almost exclusively) the legendary deluxe reverb combo doesn't sound as good to me as it does to other people!!

[redshark]

This is the bad boy......

[jelle]

Metal oxide resistors can sound pretty good.

Has the amp ever been recapped?

All voltages ok?

[jelle]

What Jbl type? Pics?

Powertubes really tired?

I am just throwing some thoughts at you, I hope it will help. Jelle

[redshark]

Metal oxide resistors can sound pretty good.

Has the amp ever been recapped?

All voltages ok?

Yes, it has been recapped, Voltages seem to be ok according to the schematic. Like 280 in preamps, 463 in power 250's in PI.

[redshark]

What Jbl type? Pics?

Powertubes really tired?

I am just throwing some thoughts at you, I hope it will help. Jelle

JBL110F 4x8ohms 2ohms.
Power tubes(general electric 6L6GC's) tested fine, 5mA apart 40mA and 45mA with bias probes.

I'm picky and I like them to be matched within 2-3mA apart and although this ones are not 5mA is not an scandalous mismatch!!

Thanks for the advice Jelle!!

Are you sure metal oxides sound good in the preamp?
I'm about to replace all the 100k's in V1 and V2 with carbon films!!

I also need to replace the EH in V1 and V2 with some RCA's.

[jelle]

I never used that brand MO resistors for plates, so they could be dull.

Try different tubes, all of them.

Those speakers should be the brightest and cleanest thing in the universe, something is up.

Is the OT toast?