Garnet BTO LB400F on the bench

[TubeGeek]

I have a Garnet BTO 400F on the bench today for servicing and I have noticed a few differences between the actual wiring inside the amplifier vs the schematic.


There is a spike protection circuit on the plates of the power tube sockets that I need confirmation on.  See attached photo.

[PRR]

> the actual wiring

Does it look authentic? If so, leave it.

The technical difference of the various values is not real significant.

[TubeGeek]

It looks authentic to me.  I wasn't sure if one way was better than the other, I thought maybe both ways worked for the purpose.  Thanks for the clarification.

[clyde]

Looks like a conjunctive filter to me.  Was Garnet using cheap transformers?

[John]

It seems like the yellow way would be "more correct" since the resistor is in between the caps. Is my thinking wrong?

[kagliostro]

Looks like a Zobel Network


Franco

[PRR]

> resistor is in between the caps

Series network. Order does not matter.

Ideally there would be just one cap. But it has to be HIGH voltage. Higher than usually stocked at the local distributor (no innernet those days!). Stacking 1.6KV to get ~~3KV was the done thing.

[TIMBO]

Photo's 

[xm52]

Gar mentioned this in his book. He used two caps because if one shorted, there is a backup. This was used in older Garnet amps.


The circuit can be replaced with two strings of three 1n4007 diodes. This is more commonly used for spike protection.

[TubeGeek]

Gar mentioned this in his book. He used two caps because if one shorted, there is a backup. This was used in older Garnet amps.


The circuit can be replaced with two strings of three 1n4007 diodes. This is more commonly used for spike protection.

I have seen this before...first time I saw it was from Ken Fisher's trainwreck pages.  This mod is actually in the Garnet book on page 86, listed as his idea for spike protection.  Nice to see different approaches.

[TubeGeek]

Photo's 

[TubeGeek]

Here is a photo of the Garnet spike protection circuitry we are discussing:

[TubeGeek]

Looking over the circuitry further I notice the stinger circuitry is not wired the same as the schematic, leaving me with some questions...  I tried to annotate this picture to show what I am seeing and make it as clear as possible to save readers time, hopefully.


First question is to do with the 0.01uF capacitor on pin 2 that is in the amp but not on the schematic.  It looks like a dc blocking cap and isn't a bad idea


Next has to do with the 4.7M resistor on pin 3.  According to the schematic, one side of the resistor should connect to pin 3 and the other side to point E.  It doesn't actually connect there in this amp, it connects to ground instead.  Is this a mistake or a correction/mod by Gar?


The owner brought it to me saying the stinger doesn't work.  I suspect this might be why.

[PRR]

Move the 4.7Meg over to E, see what happens.

[TubeGeek]

Imagine that...The stinger now works!


I can check that off the list to fix now.

[TIMBO]

So what does the "stinger" sound like 

[TubeGeek]

Well I played a guitar through the amp for a few minutes and it doesn't sound very good to me.  It gets farty. 

This will be the 3rd or 4th amp I have serviced with the stinger in it.   I fell in love with it for a few days and almost built myself something based off it and then a few days later, the thrill had dissipated.

I will plug a bass into it and a proper cabinet and then report back how it really sounds.

As for now, I am waiting for new electrolytic caps to show up so I can complete the job.

[TIMBO]

Yeah, tried it myself, it sits on a shelf looking very sad.

[xm52]

There was a later revision of the stinger circuit that you might be interested in.

[shooter]

What's the *valve* symbol on the tubes cathode R to ground

[sluckey]

That's a switch. I had to look at several other Garnet schematics to find out.

[shooter]

Thanks Steve, didn't figure we were water cooling guitar amps...yet