Cathode Transistor?

[lowell]

Hello,
I'm finishing up recapping a Music Man amp (#2475-130) that runs on 6CA7 (EL34) tubes. I noticed that this amp has something different and interesting in the output circuit. There are JE1692 npn-type transistors on the output tube cathodes. One per pair (2) and the collector is connected to cathode/emitter through 3.9ohm resistor to ground. What is this? Some sort of self-biasing arrangement? Should I replace these w/ new? Also how about the LM1458 dual op-amp... should this be replaced, and what role does the op-amp play?

[Bruce / Mission Amps]

Hello,
I'm finishing up recapping a Music Man amp (#2475-130) that runs on 6CA7 (EL34) tubes. I noticed that this amp has something different and interesting in the output circuit. There are JE1692 npn-type transistors on the output tube cathodes. One per pair (2) and the collector is connected to cathode/emitter through 3.9ohm resistor to ground. What is this? Some sort of self-biasing arrangement? Should I replace these w/ new? Also how about the LM1458 dual op-amp... should this be replaced, and what role does the op-amp play?

.... without going into a whole new typing frenzy... no unless one is leaky or damaged.
If you change one because of some reason, you really should change them both and make sure they both are fairly close matched.
This is a Class B PA and all that SS stuff makes a big difference in how they work or don't work correctly.
MOJO use to sell matched pairs...

[Rob Mercure]

These are a bit different - notice that the output tubes grids are effectively grounded as the cathodes are driven by the transistors you mention - the output Class is as close to "B" as you'll see in a modern amp. The ICs cover all of the preamp stages. There are two variants of this circuit and while I've worked on more of the other variant with a more "normal" 12AX7 split load PI none of the cathode driven amps I've worked on had defective driver transistors so if the output stage seems to be working correctly I'd leave 'em alone. These beast bias really "cold" - something like 10mA per output tube if I remember correctly (schematics are on Schematic Heaven, for instance, and the bias current is stated on them) - so don't be alarmed if the current is low and don't court disaster by increasing it <grin>.

I have had to replace a few of the LM1458s mostly due to the originals being noisy - if you've got a lot of hiss and hash I'd suspect these cans.

As a side note: I've got one of the 4 X 6CA7 chassis that was given to me in a trashed cabinet with a couple of bad speakers. While bored a couple of winters ago and lacking good replacement tubes I converted the output stage to take 6BQ6 horizontal output tubes by fabricating four plate caps and cobbling up a low voltage screen supply using transistors/zeners that I swiped from somebody's website <grin>. Really sounds great and utilizes high quality low price NOS USA power output tubes.

Rob

[lowell]

thanks guys... so is this then considered a "common grid" circuit? The ac signal comes into the output tubes at the cathodes?? weird. If anyone cares to educate me on the LM1458 dual opamp's role in this I'd be greatful.

[jrfrond]

Yes, it is a common-grid circuit, also known as a "Williamson Amp". This design neutralizes the effect of interelectrode capacitance, and was therefore used a lot in RF amplifiers.

What you will also notice with Music Man amps of this design is that the plate voltage run very high, but the tubes are well within the safe operating area because of the low bias current. The power tubes tend to last a long time in these amps.

The driver transistors are not a routine maintenance item, and need not be replaced unless defective. Those transistors are nothing special, and many other devices will work.

[kg]

Yes, it is a common-grid circuit, also known as a "Williamson Amp". This design neutralizes the effect of interelectrode capacitance, and was therefore used a lot in RF amplifiers.

while i agree that the common grid connection is/was used in a lot of RF amps, i've never heard that topology described as a "williamson" before.

when someone says williamson to me, i think of a common cathode stage, direct coupled concertina splitter, a differential amp driver, and a triode connected push pull output with a touch of NFB.

you have me curious of when his name became so associated with a CG stage!

ken

[teemuk]

If anyone cares to educate me on the LM1458 dual opamp's role in this I'd be greatful.

First opamp stage is a simple inverting voltage amplifier that mixes two signals: The input signal coming from the master volume potentiometer and the negative feedback signal coming from the output. Coupled to this stage is another inverting voltage amplifier. However, this one has unity voltage gain and therefore it only inverts the signal's phase. So, as a whole the two halves of the LM1458 form a phase splitter with some voltage gain. You can see how each half of that dual opamp drives its own half of the push-pull circuit.

[Rob Mercure]

I want to echo Ken's response - the transistor wasn't even invented when Williamson published his designs. The MM is an extremely uncommon circuit while the Williamson is pretty much the most common circuit of the '50s and '50s. The commonest mistake about the Williamson is that is it ultralinear but that UL also came later.

Rob

[lowell]

thanks everyone! Ok... so how do I bias this thing? I tried xformer shunt method and fried my meter's fuse... seems to be 1.6amps of current! I gues these amps cannot be biased this way.

[Bruce / Mission Amps]

thanks everyone! Ok... so how do I bias this thing? I tried xformer shunt method and fried my meter's fuse... seems to be 1.6amps of current! I guess these amps cannot be biased this way.

You should have had no trouible doing the OT secondary shunt method.
I suspect you shorted across the OT center tap to ground somewhere instead of lug 3 of the power tube sockets
I'm thinking 7ma to 10ma max per tube is about it.
Although not exactly like many of the grounded grid RF amps I've also worked on, I've also always assumed it was GG.
Regardless, although the transistors are not hard to find, I have worked on a few these amps and stand by my statement that you need to use matched transistors if you find a bad one.
You can change the bias of them but not balance so watch out that you don't mess up that little trim pot, which I think is for the NPN driver bias.

[slor]

Speaking of which, I'm about to dig into my bandmate's 2475-130. The amp sounds good for the most part, but recently channel two is about 30% quieter than channel 1. Besides electrolytics, are there any components that are typically swapped out during routine maintenence?

[g1]

Noisy opamps or dirty opamp sockets, either of which may solve your ch.2 volume issue if you're real lucky.

[Richwebe]

Musicman RD112, EL34.

These are a bit different - notice that the output tubes grids are effectively grounded as the cathodes are driven by the transistors you mention - the output Class is as close to "B" as you'll see in a modern amp. The ICs cover all of the preamp stages. There are two variants of this circuit and while I've worked on more of the other variant with a more "normal" 12AX7 split load PI none of the cathode driven amps I've worked on had defective driver transistors so if the output stage seems to be working correctly I'd leave 'em alone. These beast bias really "cold" - something like 10mA per output tube if I remember correctly (schematics are on Schematic Heaven, for instance, and the bias current is stated on them) - so don't be alarmed if the current is low and don't court disaster by increasing it <grin>.

I have had to replace a few of the LM1458s mostly due to the originals being noisy - if you've got a lot of hiss and hash I'd suspect these cans.

As a side note: I've got one of the 4 X 6CA7 chassis that was given to me in a trashed cabinet with a couple of bad speakers. While bored a couple of winters ago and lacking good replacement tubes I converted the output stage to take 6BQ6 horizontal output tubes by fabricating four plate caps and cobbling up a low voltage screen supply using transistors/zeners that I swiped from somebody's website <grin>. Really sounds great and utilizes high quality low price NOS USA power output tubes.

Rob

Since you by your comments you are advanced in amplifier repair I am asking you to help me with my amp.My Musicman has had a lot of use before I got it . I replaced the caps last yr as they exploded. I play hrs a day at low levels. A few weeks ago it began having intermittent static and crackling which was apparently arcing somewhere.. then one day it quit and nothing but static. I opened it up and there are two 390 ohm ceramic resistors in the rectifier circuit. one had over heated and desoldered from the board. I resoldered it and when I turn it on I have a faint signal from the guitar covered by a lot of snow type static. also the ceramic resistors get real hot after being on for about a minute. I usually play with the gain up and the volume low. this may have cause an injury to the circuits. Since the rectifier resistors are getting real hot there must be a component ahead of them that is shorting? I am not at a level to use an oscilloscope and don't have one. do you think it is likely that the JE 1692's are bad ? or maybe the LM1458's? rweberviolin@gmail.com

[Enzo]

Rich, hi, welcome to the forum. let me ask you to start a new thread for your amp. This thread has been dead for six years, and already has two amps in it.

And unfortunately, Mr. Mercure passed away some time ago.

If low voltage supply resistors are burning up, I first suspect a bad IC, which would be getting hot itself, or a shorted cap.