ACOUSTIC 165 problems

[plook]

I have an ACOUSTIC 165 made in 1981 which sounds terrible. All tubes swapped out but it still has very nasty, farty, overblown, speaker-blowing distortion on both channels. The reverb is not working either, despite trying a new reverb pan.

Any other owners of this amp here who might have a schematic, manual (showing correct preamp tubes).

Any tips, schematic, pictures inside a good one etc would be appreciated.

[selloutrr]

Is the speaker blown?

when was the last time the electrolytic caps were replaced?

Have you openned it up and looked for any noticeable signs of concern? burned marks, broken wires, or leaky caps?

PICTURES

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 1Rk0c%253D

http://www.guitar-museum.com/guitar-212 ... -Combo-Amp

http://acc.homeunix.net/images/catalogs/1980/page4.jpg

AUDIO

http://www.jokerverox.net/acoustic-amps ... -165-combo

[Firestorm]

Schematic Heaven has two schematics covering the 165. Preamp tubes are all 12AX7 except the PI (12AT7). The preamp FET they used is a bit dodgy and tends to fry eventually.

[plook]

Thanks very much for that info. Great pictures.

I recently acquired the amp in this condition, speaker is fine, tested in other amp. Nothing burnt out on the board.

Which is the PI, the tubes are not marked on that schematic. Is it the tube shown on the schematic which is split across the reverb pan, send and return?

I can't find the two schematics you mention on Schematic Heaven.

Thanks very much for the one schematic and the sound links, very powerful sounding amp.

How can you tell if a cap is leaky, do you mean physically leaking or just failing and not doing it's job?

[selloutrr]

IF YOU HAVE NOT WORKED ON AMPLIFIERS BEFORE. THEY CAN KILL YOU! If you have any ANY doubt take the amp to a tech and spend the $ to have it fixed it's not worth your life.

The tubes are labeled on the glass either eched or silk screened. The 12ax7 are the "preamp" the 12aT7 or 12au7 is the PI most likely located closest to the power tubes.

Not looking at the schematic the Phase Inverter is not feeding the reverb
the PI splits the signal to the power tubes 180 degrees per pair. find the output transformer and trace backward.

Caps - Assuming you can safely discharge the capacitors you can meter the ohm's with a volt meter.
Looking at them they have a gas leak hole. when the caps give up they bubble and push corosion out of the hole.
Paper caps look waxy and smeered.. Smeared?.. not right.

[plook]

Thanks. I asked about the preamp tubes because all four of them in this amp are labelled 12AX7, so i guess someone put a wrong one in somewhere.

You said the PI tube should be 12AT7. I thought reverb tubes were usually 12AT7.

The ebay link shows a manual in the sale of that 165, my kingdom for a scan of that.

Thanks for the voltage warning. Yes, AC and DC have got all over me in the past, so i tend to be careful and keep one hand in my pocket and use a chop stick for investigating.

It's not a nice amp to work on, the tube sockets are soldered to the PCBs and you can't get under the PCBs without disconnecting everything.

[selloutrr]

12ax7, 12at7, 12au7 9 pin tubes are all interchangable it all depends on the gain structure you need / want. you could run all the same tube for all it cares.

Most PCB based amps that use tubes moutn the sockets to the PCB. removing the PCB is art of the reason you pay more to have them fixed it's all just time on the bench.

[plook]

Has anyone got any pictures of inside the chassis of this amp. Where can i find out correct voltages for the amp?

[selloutrr]

your best bet will probably be to contact someone who is selling the amp and ask for pics. if they are really nice and comfortable being inside the amp maybe they can measure voltages as well.

[gktamps]

Hello,
Fellow Acoustic owner - I've had a 160 head since new. About 2 years ago, after it had sat idle for years, I decided to fire it up again, and found it needed lots of work.
I did a complete cap job (all electrolytics), replaced the channel switching relay (it had fried because of high voltages prior to the cap job), upped the wattage of the power supply resistors, installed higher-rated rectifier diodes (the full-wave bridge had fried many years ago and was replaced with marginal diodes), replaced the ac line capacitors, and built a bias circuit on a separate board with meter taps and pots on the back panel. I also replaced a few resistors, including the noisy plate resistors, added a ground to the heater reference resistors, and while I had the front circuit panel out, changed some of the tone capacitors just for fun, then put in a bright (too bright, sadly) blue LED for channel 2.
I did all of this with almost no tube amp repair experience, and just used it as my training tool, which was very effective. I spent about 7 months on it, doing one thing at a time. I have before and after pictures, as well as pictures from another unmodified 160, so if you want them, I could post them here or email them to you.
Regarding voltages, I measured plate voltages for biasing, but not preamp voltages. I can check my records for my measurements and post them in a day or so. I remember them being up around 584v or so...
PI tube is 12AT7. I remember seeing that 165s used 12AT7 or 12AU7s for #1 preamp tube for those who wanted less gain, more clarity. With the amp on channel 1 and adjusted correctly, there is almost no way to have a cleaner guitar amp, and though I have experimented with both tubes, I have always run the 12AX7 so I have gain and preamp distortion when wanted.
Let me know about the pics.
Cheers,
Greg