Selmer 100watt PA/4 motorboating ??

[blindboybenton]

Hi all

Well ive decided to have a look at my selmer again !! Link to schematic below

I'm abit of a newbie on the valve. I brought this amp working from ebay. When i turned it on all worked but there was some issues . so i didn't test it for too long

I replaced the 2 x 32mfd+32mfd caps and the 4 x 16mfd caps (LCR) and changed all the 25MFD caps for 22MFD + 2 x new GZ34 and 4 x new JJ EL34. have not changed the EF86.. Cleaned up the pots and stuff. Took it to a gig ( Bass guitar ) and used it Half way though it started Motorboating and blow the fuse..
Lucky i had a spare amp with me

I replaced the fuse and all seems ok.. But if i have the volume up on any channel and try to plug in the bass at the guitar jack. i might motorboat not everytime
but it you connect the guitar first and then into the amp all ok, but at loud volume it may motorboat. The amp is loud and great sound
Does not happen if volume is low

Ive tried it with another set of New matched output tubes and replaced the ECC83 with used Mazda ECC83
Ive added another 47MFD cap on the 32MFD
If i touch ( with a multimeter probe) R12/26/40/54 and connection with 22nf the amp goes into motorboating
Checked the bias ( TAD Bias Master ) all ok with a matched set. and HT480v and but across the 16MFD there are 2 with 215V and 2 with 225V ? not sure if that is relevant ..

Selmer PA 100 Amplifier Schematic

can anyone help

many thanks John

[pdf64]

Aghhh - I hate how that 'reply to thread' button deletes a 'quick reply'.
See Servicing of Eleven Selmer Amps he advises that the shared screen grid resistor arrangement can result in oscillation with modern EL34.
Maybe a replacement cap is bad; try replacing each 32uF in turn with the 47uF. Don't increase C41 above 47uF.
Replaced C42 the bias cap? The bias supply is mission critical.
Pete

[blindboybenton]

Hi PDF64

Thanks for the info. Looking a the champ site. I can think he is saying using EL34B's. I had a set of JJ's and Electro Harmonixs all matched and the amp still did it. I dont have those valves to hand now. But i do have a mixed bag of old Tubes.. I know they are not matched, But the amp did it with them and with new valves

Ive replaced the Bias cap. If one of the new caps i replaced are down.would adding an extra cap across it help in the diagnoses ??. If i go down the route of replacing the caps again. Should i up the valve of the 16MFD to 32MFD ?

many thanks for the help..

[pdf64]

Yes, you could put a good high voltage cap across each in turn to see if it stops the motorboating with one of them. But don't do it on a live amp unless you are competent and confident - there would likely be sparks! And don't increase the reservoir too high.
Modding your way out of a fault should be the last resort; there is most likely a bad part, bad ground - have to assume that the design is good.
Pete

[blindboybenton]

Thanks PDF

Ive checked everything. Nothing is wired incorrectly. so ive bitten the bullet and ordered all new filter caps.. I will post the outcome

BBB