Old Gibson Amp

[Pryde]

I have a nice fella who brought me an old Gibson Hawk GA-25 RVT that actually sounds pretty good as is. It has ALL ORIGINAL RCA tubes. The amp was his dads and now he is giving it to his 9 year-old son who is learning to play the guitar....pretty cool.

Schematic here:
http://www.classictubeamps.com/schem...0schematic.pdf

He says to do whatever it takes to make it "tip-top"

I installed a 3-prong plug, cleaned pots, tube sockets, all jacks, installed new speaker wire as the old one was deteriorating. Visual inspection looks like everything is brand new inside. I think this is the first time it has seen day light since built.
Plate voltage is 320 with bias current at -36mvDC on original tubes. Seams reasonable but bias is kind of high for 6v6's I suppose?

The amp is relatively quiet, so I don't suspect bad filter/coupling caps but they are very old and original (no discharge or bubbling)

What would you also do in a case like this? I don't want to overlook anything important that might need attention. etc. How to do folks service an old amp like this? Thanks for the insight.

[The Dude]

You should also clip out the death cap.

[Pryde]

You should also clip out the death cap.

Yes that was done when updating to the 3-prong plug.

[Jazz P Bass]

"How to do folks service an old amp like this?

Clean the sockets, pots & jacks with deoxit.
Verify that the Vdc & the Vac ripple is within reason on all power supply nodes.
Verify that any & all coupling caps are not leaking Vdc into the next section.
Check the bias & overall current draw from the mains.
Let er rip.

[drewl]

That pretty much nails it.
Make sure it's safe and operating correctly, clean and retension tube sockets, replace any bad caps or resistors that have drifted out of spec.

[Pryde]

"How to do folks service an old amp like this?

Clean the sockets, pots & jacks with deoxit.
Verify that the Vdc & the Vac ripple is within reason on all power supply nodes.
Verify that any & all coupling caps are not leaking Vdc into the next section.
Check the bias & overall current draw from the mains.
Let er rip.

THanks Jazz. Do you always scope the power supply nodes to determine ripple or just DMM for amount of DC or AC present?

[Jazz P Bass]

THanks Jazz. Do you always scope the power supply nodes to determine ripple or just DMM for amount of DC or AC present?

Fluke DMM.

[J M Fahey]

replace any bad caps or resistors that have drifted out of spec.

Sorry but this kind of blanket statement usually opens a big can of worms.

1) to properly do it, at least 1 leg from every component must be lifted so you can measure it.

That alone messes with and stresses every part.

Not forgetting that this amp is probably built on terminal strips with legs twisted around terminals and inside holes.

2) parts have tolerances.

Even when new and much more 60 years ago, when 20% was the norm.

I can foresee long threads full of questions such as: " the plate resistor, labelled 100k, actually has 118k ... or 87k ... keep/replace?"

3) so I suggest playing the amp, checking basic voltages, all what JPB suggested in post # 4 and leave it as is, unless a real failure shows up.

But as even Leo printed on schematics, "anything within 20% is fine" .

And in some points even larger tolerance is acceptable.

Kudos to pryde's customer for keeping Grandad's passion alive

[Pryde]

Sorry but this kind of blanket statement usually opens a big can of worms.

1) to properly do it, at least 1 leg from every component must be lifted so you can measure it.

That alone messes with and stresses every part.

Not forgetting that this amp is probably built on terminal strips with legs twisted around terminals and inside holes.

2) parts have tolerances.

Even when new and much more 60 years ago, when 20% was the norm.

I can foresee long threads full of questions such as: " the plate resistor, labelled 100k, actually has 118k ... or 87k ... keep/replace?"

3) so I suggest playing the amp, checking basic voltages, all what JPB suggested in post # 4 and leave it as is, unless a real failure shows up.

But as even Leo printed on schematics, "anything within 20% is fine" .

And in some points even larger tolerance is acceptable.

Kudos to pryde's customer for keeping Grandad's passion alive

Thank you JMF. I was scratching my head about who has the time to go through and check every cap/resistor in amp for correct tolerance

I think the amp would likely benefit from new filter and cathode bypass caps as they are so old (1963) but if they look and sound ok then why bother right? I guess in this case I will advise him to keep everything stock (tubes and caps) and just play it until something fails.

I will go through and test everything per Jazz P. Bass's post.

[hylaphone]

I think the amp would likely benefit from new filter and cathode bypass caps as they are so old (1963) but if they look and sound ok then why bother right?.

Replacing the electrolytics is preventative maintenance, why not take care of this while you are in there? Especially if he wants it "tip-top" for his son...
It's like driving a classic car with the original tires.. it's cool that they are still on there, but why run the risk of a perilous blowout?

[pdf64]

Analogies between amp and automobile maintenance just don't sit right for me.
As long as an amp has a suitable safety ground, line fuse and is in its cabinet, it poses little risk of personal injury.
Caps can explode, tubes short but the only harm will be to the wallet.
Whereas an automobile has a myriad of safety critical and safety important components and systems (the most troublesome being the one behind the wheel).

[J M Fahey]

I was scratching my head about who has the time to go through and check every cap/resistor in amp for correct tolerance

Look no further than:
diyAudio

Lots of loonie and time wasting ideas running there.

[g1]

Analogies between amp and automobile maintenance just don't sit right for me.
As long as an amp has a suitable safety ground, line fuse and is in its cabinet, it poses little risk of personal injury.
Caps can explode, tubes short but the only harm will be to the wallet.

Ok, how about a classic car with the original oil then?

[pdf64]

Even then, if the engine seized whilst tearing down the highway, or turning across traffic, bad things could happen, the consequences could be far worse than just having to deal with the mechanical breakdown.

I remembered an additional safety important system of an amp - its carrying handle/s.
If that breaks loose then maybe a crushed foot or a tumble down stairs.

[hylaphone]

Changing oil and dry rotted tires is also preventative maintenance.
Dude, good luck with the Gibson. That is one lucky 9 year old.