Hi guys. Im working on a 50 year old Sona GS20 amp. Loss of volume issue.

[highlux]

Hi there. I'm helping a friend repair an amp.

The cliff notes:

The amp has low volume. a 1/5 of normal volume as an estimate. The amp sounds ok. No popping, hissing, humming.etc.
Treble and Bass pots work. Trem and reverb work. (I assume the preamp side is ok from this?)

1960 Sona GS20 I have schematic from a Hohner Contessa...the same amp, just private labeled for Hohner.

Working on getting full set of new tubes.
Have 3 prong grounded cord on the way.
Have not tested voltages as of yet. (Will need guidance here...as I am an amateur at best.)

A visual inspection reveals nothing obvious.
All tubes glow when on.

I will get some pictures up later today.

Thanks in advance.

[highlux]

[ThisLifeILead]

the schematic has the tube pins marked so just follow the schematic for voltages. pin 7 on the el84/6bq5 tube are the plates. Go through and check for voltages. it is cathode biased so you can put in a new set of tubes without too much worry but i highly suggest checking the bias when you do. it easy and when you get to it we can help guide you. low output can be alot of things but doing voltage checks will help rule some prelim stuff out.
When measuring voltage as a noobie, put one free hand in your pocket for shock safety so your not tempted to rest it on the chassis. set your multimeter to the dc range (straight lines) and make sure your amp is connected to a speaker. let us know how it goes!

[highlux]

Thanks man! Looking forward to working with some cool amp people. I am going to solder on a new grounded cord before I measure voltages etc. I have it ordered, should be here in a day or so.
I know we are at a standstill until that information us supplied. But since I'm a noobie..I want to be as safe as possible and not develop any bad habits re safety from the get go.

[ThisLifeILead]

you can measure voltages with the original 2 prong if its in good shape. but if the wire its brittle and cracking at all, then wait.

[g1]

Meanwhile read up on high voltage safety precautions for amplifiers, and discharging capacitors.

[highlux]

Its cracked at the connection to the plug. Usually I wouldn't pay it any mind....but the DC voltage has my respect. This thing is 50 years old.
I'm so glad to have so help to guide me through. I have a rubber mat in the garage workshop on the floor. And I know about the one hand deal so
as not to complete a circuit that includes my heart...lol.

While waiting for the cord...I have repaired the tattered grill cloth...got lucky there was enough material rolled over the edge to make it work. Soldered new leads on the speaker
wires. Cleaned the pots and tube sockets. It has 2 new power amp tubes....3 out of 4 pre amp tubes are original. I have swapped know good ones in and out to no avail. They all light up.

[highlux]

Meanwhile read up on high voltage safety precautions for amplifiers, and discharging capacitors.

Will do. Correct me if Im wrong....but from what I understand...I can connect a lead to ground...and to pin 1 on any pre amp tube to discharge caps. I also have a
lead with a resistor soldered inline to directly discharge filter caps as well. (made that for a solid state amp I re soldered a loose filter cap joint on)

I don't mind erring on side of safety...since I am an amateur.

I will follow instructions as given. I'm pretty handy and am a quick study. Lots of tools.

I will post some pictures here later on today. Think I'm gonna go jam a little bit. Got a new to me amp last night. Straight out of the 70's. A crate amp...when they were made like an actual crate! Doesn't sound terrible...lol

Im going to sandblast and powdercoat the footswitch today. Its hard wired and in ROUGH shape. I need to get 2 new switched for it as well...ones like a stomp box has.

[52 Bill]

Will do. Correct me if Im wrong....but from what I understand...I can connect a lead to ground...and to pin 1 on any pre amp tube to discharge caps.

Yes, that will discharge the caps through the plate resistors. There is what amounts to a basic bleeder resistor (R47 and R48) across the power supply that should discharge the filter caps if you wait a few seconds, but you should check with your meter before assuming that the caps are drained.

There are a number of simple tests that you can perform before you change the power cord. I would suggest that you check the power supply resistors R44, R45 and R46, as well as the cathode resistor for the power tubes R25.

[highlux]

Sure...am i checking for a certain voltage value? resistance? I'll follw your instructions to a T. And Thank you!

[highlux]

Too late....I fixed it. Was a bad brand new power tube. Plays like new.

Thanks...I will check back on the next one...I have a Peavey 30 classic to look at.