amp teck help requested

[Jay101]

Jtm 60 I am having power out issues with this amp i had a bad rectifier swaped him out and swaped tubes with other tubes they are all questionable i suppose pin outs on the tubes read as follows 2-7=6.97 vac ,3=482 vdc,4=479 vdc,5 = 38.7
to the OT i have 20vac in and am gitting 300mv out is this correct do i just have bad tubes , any advice greatly apperacited...101

[Albert Kreuzer]

I have had this amp that i got used for about 2-3 years decided to change tubes and did so. man told me how to bias it with a multimeter???? done what i was told by checking across the only pot on the power board that shorted something tubes went extra hot i killed the stand by or power i cant rember fired it up again poped fuse.... got my self a rash of spares and started my novice tech work... turned down the pot and re tubed her had a slight buzz for several hrs but worked rather well. maby played 20hrs oh i should mention i have had high pitch feedback that is uncharacteristic of this rig...then all of a sudden it dies. well not exactly i can hear maby 2% volume all clean/dirty reverb ect seems in order it just has no power.main transformer seems ok i swaped power tubes with others and have the same condition.. caps,rectifier,heater????? any sugestions graciously accepted.Thanks in advance..101
agree that i probly goofed with the help of some bad advice and am verry intrested in learning how to repair this myself with help from willing people who know amps i do exelent solider work and know fair amount about other eletronics no parts in there i cant replace for a few dollars..i used to know this great teck but some how lost him. my labor is fee to me plus i enjoy the education. any parts to check or formatable test to run on them will be greatly appreciated.thanks in advance for any help

i pulled this from a previous thread.. also i replaced the rectifier it had a bad leg but didnot fix the amp main transformer voltage checks ok thanks for any help...101


Jtm 60 I am having power out issues with this amp i had a bad rectifier swaped him out and swaped tubes with other tubes they are all questionable i suppose pin outs on the tubes read as follows 2-7=6.97 vac ,3=482 vdc,4=479 vdc,5 = 38.7
to the OT i have 20vac in and am gitting 300mv out is this correct do i just have bad tubes , any advice greatly apperacited...101

Hi Jay,

we will better be able to help you if you keep this all in one thread instead of starting new ones and sending PM's.

Voltages on the power tube pins look OK, except that this should be -38.7 on pin 5.
What do you mean by "20VAC in and 300mV out" at the OT? Are that signal voltages from a tone generator?
Could be your OT is toast.

[Jay101]

ok i thought it best to start a new thread my mistake. thanks for your post ill try to better my forum ediqute. that voltage was in and out of the ot using a guitar for a signal connected as normal. i also tried jumpering eff. loop again and pluging the guitar in there also neither had any benifet thanks again.

[Albert Kreuzer]

Hi Jay,

if you measure 20VAC across the primary OT (plate-to-plate) and at the same time get 300mVAC output, the OT is kaputt.
If you measure at the output jack, it could also be a bad impedance selector or a bad jack. Or a bad solder connection in between.

A guitar usually does not provide a steady signal (unless you play it with an e-bow). Better use a signal generator.

HTH
Albert

[Jay101]

thanks albert

[Jay101]

this amp has el 34,s can i use 6l6 or some other tube in its place??? if so what would be the benifits or disadvantages???? thanks 101

[Jazz P Bass]

It appears that your output transformer is bad.
Personally I would use a different transformer for EL 34 & a different one for 6L6.

[Albert Kreuzer]

this amp has el 34,s can i use 6l6 or some other tube in its place??? if so what would be the benifits or disadvantages???? thanks 101

You can convert the amp to take 6L6 (or KT66/77/88 or 6550), but you have to check the bias voltage and possibly change the bias circuit.
Usually El34 need -38 to -42 Volts, while 6L6 typically are more in the -45 to -52V range.

As Jazz P Bass stated, you will get a slight impedance mismatch with the OT, but nothing to worry about.

No special benefits or disadvantages, just different sound and feel.

Cheers,
Albert