ab763

[selloutrr]

I'm distracted by my 4yr old so i'm making a game plan before i go in.

Issues:

the reverb hums and squeels.
the tubes are a tad on the low side.
I can get the hum to go away by tube swapping but then anything over "4" on the reverb volume makes the amp squeel.

Intermittent Crackling when playing the E string. (similiar to the sound of plugin in to a powered on amp)

[Bob-I]

the reverb hums and squeels.

Common problem. It's usually lead dress or the reverb cables. Check out some pics of a Super Reverb or other AB763 and copy the lead dress. Fixed mine.

the tubes are a tad on the low side.

I assume you mean bias on the low side. Increase the 470R on the bias supply board until you get the bias range correct. The C- voltage should be around -50VDC.

Intermittent Crackling when playing the E string. (similiar to the sound of plugin in to a powered on amp)

If the bias is very low this will happen, fix the bias problem first then see if this is still a problem.

[selloutrr]

Thank you,

The wiring hasn't changed in the amp since it was made but the hum issue is new as of 2 weeks ago. I'll poke around, but the squeel leaves me thinking it's something that can fail.

sorry.. by a tad low on the tubes I meant I tested them and they are in the mid 70-low 80 percent tube life. I personally like replace tubes under 85%. The output bias is about 70% on the output tubes.


I'm looking at doing a cap job as long as i'm in. anyreason i can't use some 500Volt filter caps i have in stock to replace the 550Volt ones under the can?

[selloutrr]

squeel was from a bad tube and corrossion on the footswitch jack.

crackle probably is the mismatching sides of the Phase inverter tube forcing half the tubes to run colder then desired.

the hum... soon to be silenced with a cap job and 3 prong power cord ( bypass of ground switch and death cap removal.

[selloutrr]

do to space is there any issue with using a 500V cap in substitution of the 550V. 600V don't fit under the cap can.

[Tinkerer]

Can't see a problem with using 500V caps. I used 450V caps when I built my Black Face Twin and I haven't had a problem.

Tinkerer

[M Fowler]

I don't see any problems either and I use IC 500v on my Fenders.

[selloutrr]

some pics

Changed out the 40 year old electrolytics.
Added a 3 prong grounded power cord
Removed the Ground switch and Death Cap from the Circuit
Installed an MOV across the curtisy power

Increased the Bias resister from 15K to 23K
Biased the amp for 11watts per tube (4 x JJ/Telsa 6L6GC).

Replaced the Phase Inverter and Reverb Driver 12AT7 tubes.

Basic socket cleaning/ retension, tightened all bolts and screws.

[ChrisM]

11W per tube seems pretty low, yeah?

[Phil_S]

11W per tube seems pretty low, yeah?

Ought to be about 2x that. We are talking 6L6, not 6V6, aren't we? What is plate voltage and grid voltage?

[C Moore]

Can I ask Two Questions?
1. What is the cap for? The one between Hot and Neutral on the convenience outlet?
2. On my schematic, the dropping resistors for B+ are 1k and 4k7. The pictures for this amp show (I think) 2k2 and 10k. Am I getting this wrong somewhere?
Thank You

http://www.schematicheaven.com/fenderam ... _schem.pdf

[C Moore]

Can I ask Two Questions?
1. What is the cap for? The one between Hot and Neutral on the convenience outlet?
2. On my schematic, the dropping resistors for B+ are 1k and 4k7. The pictures for this amp show (I think) 2k2 and 10k. Am I getting this wrong somewhere?
Thank You

http://www.schematicheaven.com/fenderam ... _schem.pdf

[selloutrr]

The cap looking thing on the power lines is an MOV it acts like a surge supressor in the event of a voltage spike.

I test voltages based on the schematic and everything was spot on or within 10%

Typical AB1 6L6 bias should give you a window of 8-13W per tube
about 410V if I remember off the top of my head.

I don't know who hacked up the amp in it's earlier life but the owner wanted to leave as much of it untouched as possible and spend the least they could. (surprise) honestly I probably could have spent all afternoon in this thing rewiring and replacing parts that where off spec. I focused on the issues at hand and worked with the budget I was given. Tried to make it the best it could be with what was left.

[Randy Magee]

A 6L6GC tube is rated for 30 watts... 11 watts is less than 37% of that... sounds real cold to me too...

[ChrisM]

Maybe he is using the non GC version?

Otherwise 11W is super cold, less than 50%.