Original GA5

[Skelt]

Hi guys,  this question causes fights on the hifi stereo forums I hope it doesn't start one here but here it goes.

My friend brought me over one of these amps to work on. The problem is it breaks up too early.   I believe the 2  caps(.02uF) in the signal path are leaking current bc an ohm meter shows resistance and never indicates an open.

So what are recommended caps to keep it as close to the original sound my friend wants?  Or Is there such a thing?

 

6V6, 12ax7, 5y3, and all resistors test good. The PS was recapped by someone else. The amp has an alnico speaker. 

 

[badbluesplayer]

You could try measuring the DC voltage on each side of those two coupling caps and see if there is any substantial amount of DC on both sides.  They should have like hundreds of volts on one side and something like one volt or less on the other side.  something less than 1/3 or 1/4 volt is usually considered "good".  If the volume control sounds scratchy across it's entire range, not just scratchy in one place, then the coupling cap before it - the first one - is leaking DC to the volume control.  That would indicate that the cap should be replaced.  Measure DC right at the volume control lugs to verify there isn't much DC on the vol control.

Another thing you could do is check the voltages on the grids of the tubes following the caps to see if the DC is leaking to the next tubes.  Same idea.  Measure voltage on pin 2 of V1 - the 12AX7 - (right after the volume control).  It should be less than a volt.  Also measure the voltage on pin 5 of V2, the 6V6.  It's right after the other cap.  It should be less than a volt, preferably less than 1/4 volt. 

This will tell you if you're on the right track.

 

Edited January 17, 2021 by badbluesplayer

[Skelt]

There's .5v at the screen of the 6v6 and .3 at the volume pot. Ill try to convince my friend they need changed.  What effect do the cathode caps have on tone?

I put a .022uF 200v cap in at the 6v6 temporarily and it sounds somewhat better. Im using a phone for a signal as I am no guitar player.   Ill order some in 600v. 

Any thoughts on changing the cap at the volume pot to .016 uF? I read another post that the op said it reduced some muddiness.

[badbluesplayer]

On 1/17/2021 at 7:15 PM, Skelt said:

There's .5v at the screen of the 6v6 and .3 at the volume pot. Ill try to convince my friend they need changed.  What effect do the cathode caps have on tone?

I put a .022uF 200v cap in at the 6v6 temporarily and it sounds somewhat better. Im using a phone for a signal as I am no guitar player.   Ill order some in 600v. 

Any thoughts on changing the cap at the volume pot to .016 uF? I read another post that the op said it reduced some muddiness.

O.K.  The coupling caps are good with that little voltage leaking through them.  You can lower the values of the coupling caps to make the tone brighter overall.  Just like you said, start with the first one.  The earlier in the circuit, the more effect any change has on the overall tone, so start with the first one.  You can lower it to .016 - I'd just try 0.01.  I would use Mallory 150 capacitors for those.  600V rating.  Like these -

https://www.amplifiedparts.com/products/capacitor-mallory-630v-150s-axial-lead

Get 2 and try the first one first, then try the second one.  See what happens.

Make sure you've tried swapping out the 12AX7 and 6V6 for other known good tubes first, to make sure it's not a tube issue, first.  Then do the coupling cap thing.

Cathode bypass caps allow high frequencies to drain away at each gain stage.  They make the tone bassier and can make it muddy when distorting.  try reducing the values at each location to  make it less muddy.  Start at the beginning.  The cathode cap on the power tube does the same thing.  Makes the amp thicker and bassier.  Lessen them or eliminate them to make the tone brighter.

I'll see what records I may have of GA-5's I've worked on to see what we did.  We've revoiced a few like yours.

👍

 

Edited January 24, 2021 by badbluesplayer

[Skelt]

All the tubes tested ok on a Lafayette KT-209. And changing tubes was the first thing we tried. 

I had an order placed with digikey for a different amp project (phono preamp).  So I bought two 630v. .022 CDE films and installed them. Now there is no dc on the low side of the coupling caps.  

Last night the owner stopped by with a 79 strat and a Gibson LP with humbucker PUs.  The Gibson definitely has the hotter PUs.

We tried a .011 uF at the volume pot and the sound was too lean.  Then we rolled several tubes thru the 12ax7 position. Now he is taking  with him a

12AU7 Sylvania(tame and mellow)

12AX7 GE (sharp and bitey)

12AT7 Sylvania (a little edgy, somewhere in between the others) 

His old RCA 12ax7 was very dull when compared to the others.

After a couple of hours of playing  the new caps are settling in. It has fixed the early breakup issue the amp was brought in for.  Now he is able to play at full volume on the amp and guitars.

Most of the volume comes in the fist half of the volume pot and after the 12o'clock position there is more added distortion than added loudness. Is this normal for this amp?

Any thoughts on adding an in rush current limiter to the power cord?

The Dynaco forum recommends to put one, a CL-80 in their case, in old tube equipment.  Supposedly it has two functions. First it brings the source voltage up slowly so the heaters dont experience full current when cold and  it knocks the line voltage down a few volts because today's 120v line is higher than it was in the 50s & 60s when it was more like 110-115. 

[badbluesplayer]

Yes.  Sounds like it's working pretty well.  It's normal for those to distort at 12:00 and just get more distorted.

I've never put an inrush limiter in an old amp to get the voltage down.  You could.  A better way to get the voltages down would be to increase the resistance of the one or more of the dropping resistors.  But the plate voltages are pretty low on that amp.  A 10% wall voltage increase won't be a big deal.  Those amps kind of like to run hot anyway.  I'd try it like it is with the thought that you could increase the dropping resistors, like from 10K to 15K on the first one.  I'd leave it for now.

[Skelt]

I checked the heater voltages 4.97 and 6.45.  so no worry about the line being higher today compared to the past.

Thank you bluesplayer for your help, this is a very cool circuit. I can see why these are so desired. Single ended with very little circuitry to get in the way.

In the future I might build a modified stereo pair to power a dedicated vinyl setup with external speakers.

But in the mean time,  how do I eliminate the hum from this phono amp?

[badbluesplayer]

On 1/28/2021 at 6:53 AM, Skelt said:

I checked the heater voltages 4.97 and 6.45.  so no worry about the line being higher today compared to the past.

Thank you bluesplayer for your help, this is a very cool circuit. I can see why these are so desired. Single ended with very little circuitry to get in the way.

In the future I might build a modified stereo pair to power a dedicated vinyl setup with external speakers.

But in the mean time,  how do I eliminate the hum from this phono amp?

Some of those had a single filament wire and they used the chassis as the other filament lead.  I have had success rewiring them to add a second lead and use it instead of the chassis.  Add an artificial center tap using two 100 ohm resistors.  Then you have a conventional two-lead filament arrangement that will run much quieter.

[Skelt]

Its a home brew winter project using parts I had gathered from years of collecting.  The PT was taken out of a webcor RR.  It has a 6v heater winding and using a 6X5 rectifier there is  450v B+.    The 12ax7s heaters are supplied by a 12vdc wall wart. 

Its built on the lid  the RTR the PT came from.  If I can quiet it down and like the way it sounds then ill build a better enclosure and use new tube sockets and 1% parts.   With this build I want to try different RIAA feed back circuitry. 

I can see the hum on a scope and it goes up and down with the volume. There is about 50mV ac at the output.  I'm going to disconnect the input and output wires to the rca jacks and move the PT far away.  Metal sheilds around it and the signal tube hasn't done enough.

Edited January 29, 2021 by Skelt

[Skelt]

an update on this GA-5

Well its back in the shop, the 1A fuse wont hold. pulled the tubes and the same results. the tubes test within spec 

a 2amp fuse will hold but the PT begins to get hot and start to sizzle after about 30 seconds to a minute

i see no arcing on the chassis, 

The owner say he use this amp about 18 times in the 3 years since I worked on as described in the top this thread.

im about ready to pick one up from mercury magnetics unless im missing something

 

Edited Sunday at 10:20 PM by Skelt