Gerhart/Guytronix?

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Lucian
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Gerhart/Guytronix?

Post by Lucian »

Hi All,
I have successfully completed a Valve Jr mod which sounds great and I owe it to this site and all of you tone enthusiasts. Till now, the last time I picked up a soldering iron was in cub scouts in the 70's.

Anyway, since I rarely gig anymore and spend most of my time writing/recording, an 18watt might be too loud for me to use so as a first complete kit I was considering a Gilmor Jr as it is .5 watt and a differs from a class a SE like Epiphone: "The 6n1p output is configured as self-split push pull."

Has anyone have any opinions/experience with this amp as I am curious how a push-pull differs from a Single-ended design as I know nothing about any of this. Thanks :)
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Phil_S
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Post by Phil_S »

I built it. I have it. It's a heck of a good value and a fun project. If it's too dirty with the 6n1p, you can wire it for an ECC99 and that will give you 2W. I did mine that way. Rich sells a tube adapter, so that you can go back and forth on a plug 'n' play basis. There are alternative tone stacks "F" "V" and "M". I built mine 3 years ago, but recollect building the M tonestack.

Since this is the "M" 18W group, you might really like the 6n1p, but I found it too buzzy and it has virtually no clean headroom. Looking back, I think that Gary must have had the 18W vibe in mind when he designed it.

This amp allows you to crank it up and still have a conversation going on in the room. You can also drive a 4x12 and keep up with a drummer if the stack has efficient speakers.

I've talked with Gary Gerhart by phone (in the days before Guytronix was distributor) and he really does know his stuff. A nice fella, too. And I've exchanged a bit of email with Rich Guy, who is also super. I understand that orders get filled quickly now that Rich is managing distribution.

Self-split is the designation because you are using two halves of a triode as if they are two tubes, and the design of the PI is related to this, as I understand it. I can't post the schematic because it is proprietary and Gary's property.

Simply put, in a SE amp, the power tube is "on" 100% of the time. Because there is only one tube, there is no PI stage.

In a PP amp, each tube conducts half the signal in alternating fashion, so it's "on" 50% of the time. You need a PI in a PP amp, to invert half the signal and send it in alternating sequence to each tube. Many think the character of the tone is different from a SE amp, but I can't articulate in writing on this point.

The GJ is a great sounding amp.

If you decide to get a GJ, tell Rich I sent you.

Regards,
Phil
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jnewman
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Post by jnewman »

I could be wrong, but I'm under the impression that self-split push pull doesn't have anything to do with the fact that you're using two halves of one tube as the power section but instead means that there's no phase inverter before the power section. Is this not right? For example the firefly, which has the signal going only to one triode's grid, with the other triode's grid grounded and both cathodes connected together and then through a common resistor to ground.

I've seen a couple amps with a 2xEL84 output stage that were called self-split push-pull amps.
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ontariomaximus
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Post by ontariomaximus »

The EL84, with its high gain, would be an ideal candidate for self-split. I guess the key word is "candidate" I might just have to try it with perhaps a plexi type pre, wreck type pre, 2204, or a slo100. Thanks!
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Post by Phil_S »

I did not describe selt-split correctly. Jnewman is correct about what he says. I was too lazy to pull the schematic last night, but I've got it in front of me now. Indeed, there is no PI, which is what the term "self-split" implies. Silly me, the PI is sometimes referred to as the phase splitter. I am sorry for posting the wrong info and thanks for catching it.

The way the GJ is wired, there is one dual triode preamp, providing two gain stages. From the second gain stage, there is grid input to one half of the power tube (a dual triode), the cathodes are tied together and grounded through a resistor, each plate goes to one leg of the OT, and the grid on the other half is tied to ground.

I believe the real secret to the GJ lies in the output transformer, which is a Gerhart proprietary design wound by Mercury Magnetics. You can't purchase this OT unless you get it as part of a Gerhart product. I think he uses the same tranny in the Ardmore.

Here is a good place to find out more:
http://www.gerhartamps.com/gilmorejr/bu ... ilders.htm
http://www.gerhartamps.com/products.htm
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Phil_S
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Post by Phil_S »

Here is an example of a self split PP EL84 amp:
http://amps.zugster.net/projects/pa20
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ontariomaximus
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Post by ontariomaximus »

Re: Self-split - can a second preamp feed the other EL84's grid instead of going to ground?
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Phil_S
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Post by Phil_S »

I'm not an engineer, so this is speculation on my part, but I think you are describing either your typical two section PI, or two SE output tubes. I don't see how you can do what you suggested. Anyone else?
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ontariomaximus
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Post by ontariomaximus »

I like the idea of 2 separate preamps feeding opposite sides of a LTP, and I don't see any reason why you couldn't do it with the self-split output stage. Can anyone else clarify?
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Post by hucky »

ontariomaximus wrote:I like the idea of 2 separate preamps feeding opposite sides of a LTP, and I don't see any reason why you couldn't do it with the self-split output stage. Can anyone else clarify?
you can feed the 2 indputs of the selfsplitting poweramp - but in general it will not simplify or advance the basic circuit.

with selfsplitting poweramp you get:
- the poweramp is working 100% Class A in pushpull. the 2 powertubes are conducting current in positive and negatibe wavecycles
- you get local cathode feedback on both powertubes and less gain in the poweramp
- you get less output power. 10 watt maximum for 2xEL84 or about the same as 2 EL84 in single ended parallel.
- you have to adjust the OT for lower primary impedance (Hammond 1604 working with 8 and 4 ohm instead of 16-8-4 Ohm. (hi-Octane)
- you get high bias current in powertube (class A) and more hum if you dont care powersupply ripple and OT primary winding symmetry
- if you take care you can get nice clean and overdrive sounds but not high power with the same tubes compared to standard phasesplitter class AB amp.
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ontariomaximus
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Post by ontariomaximus »

I am thinking of a 10wpc, 2 channel amp with LOW parts count. Does a 4k primary sound right for a pair of EL84's? I am just curious why the difference in primary impedance. Thanks for your post.
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Michael
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Post by Michael »

8k primary is "optimum" for common AB1 PP configurations, not 4k. However, you can always optimize using either multiple secondary impedance taps or by changing speakers to optimize reflected impedance.
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ontariomaximus
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Post by ontariomaximus »

The previous poster stated a lower primary Z is needed for a self-split output stage (no cathode bypass cap). I was wondering why and if 4k is appropriate.
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Michael
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Post by Michael »

ontariomaximus wrote:The previous poster stated a lower primary Z is needed for a self-split output stage (no cathode bypass cap). I was wondering why and if 4k is appropriate.
Man, I'm just off tonight, not reading entire posts. Sorry.
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