Gretsch electromatic / Champ 600 mods

[Colas LeGrippa]

Hi,

has anyone performed a mod on a Gretsch electromatic / Champ 600 guitar amplifier ?

[Colas LeGrippa]

........looking especially for Fromel's mod, thanx

Colas

[HotBluePlates]

........looking especially for Fromel's mod, thanx

Colas

Fromel doesn't list either of those amps on his website, so don't know where to start.

The description of the Valve Jr mods on that site don't sound like anything too esoteric, but simple changes that you'd know how to do based on amp characteristics you'd like to change (like adjusting bass response, rebiasing preamp/output stages, etc).

[Colas LeGrippa]

Hi !

Fromel doesn't list either of those amps on his website, so don't know where to start.

Well yes, there's the Champ 600 listed further down the list if you put your glasses on,  you'll see
Now, the problem is that I don't want to pay for a kit including the schem since I have billions of parts.
I was just wondering if someone had performed mods proven to improve the tone of the Champ, Fromel or other and would have liked to share the schem.

Have a nice w-e

Colas

[HotBluePlates]

How about that! It's mixed in with the pedal mods, which is how I overlooked it.

He doesn't tell you much on that page, so assume the changes are similar to what he does on the Valve Jr. He actually tells you basically what he does with that amp:

New coupling caps, change preamp cathode bypass value, change preamp and output tube cathode resistor values, add a bright cap to the volume control, New first filter cap and output tube bypass cap, new input jack, ultra-fast rectifier diodes and tell you how to route the wires in the amp.

No magic there, and can either be found in others' mod suggestions for the Valve Jr, or out of your own head for what works in an amp.

[Colas LeGrippa]

I have built this Champ yesterday with PT and OT I had. I have tried to mod it for hi/mid boost, bypassing the second half cathode resistor with a .68 cap. The circuit did not like it. The  amp started to motorboat with the volume knob turned to three quarters and up. I inserted a small ceramic cap randomly here and there in the circuit to remove oscillation but unsuccessfully. I removed the .68 uF bypass cap and everything got back to normal. At full volume, a nice overtone feedback whistles happily in my ears. To increase gain , I will bypass the second cathode resistor with a 25/25 and see what happens. I can figure out the .68 was too trebly to prevent motorboating. ( but in my P/P, EL34, the .68 does the job great ! ).
I have hooked up a multi position rotary switch with different values of caps , from 50 to 500p, right on the volume pot between outer leg and wiper, and hooked up a fat boost on the first half of the 12ax7 cathode, with a 1.5k and a 330uF in parallel with the existing 1.5k and 25uF which does a good job too.

What Champ circuit do you guys prefer ? 5E1, 5F1......?
Colas

[HotBluePlates]

Unhook the feedback from OT secondary and see if that oscillation goes away.

When you put the cap across the cathode resistor, you messed with the feedback circuit. It should have shunted highs out of the feedback loop and very much boosted them (like a fixed presence setting). That you got oscillation suggests either a wrong polarity of the feedback (just like most folks run into with push-pull amps), or wiring issues causing oscillation apart from the feedback loop.

Your amp shouldn't motorboat either, as that's low frequency instability, where you normally think of oscillation as high frequency instability.

Assume the amp was stable: why not just change the 1st gain stage cathode bypass to alter the amp's sound?

[Colas LeGrippa]

Unhook the feedback from OT secondary and see if that oscillation goes away.

the feedback was never hooked up..........maybe I should hook it up to prevent oscillation, what do you think ?

one thing I forgot to say is that the PT was way to high and instead of running the 6V6 at 400 V - 450 V, I plugged in a 6550. Voltage dropped to 350V, biased at 104mA. What I dont understand is that this PT is part of a Champ kit I bought right here from Doug. If we talk about primary impedance, 350V / 104mA = roughly 3.5k. The OT I have chosen here is a MM 4.5k that gives good results even if the match is not perfect.

Thanx
Colas

[HotBluePlates]

the feedback was never hooked up..........maybe I should hook it up to prevent oscillation, what do you think ?

No, then you have some other type of wiring problem.

But a 6550 at 104mA sounds pretty steep on a Champ PT. Is that the 125P1B that Doug sells? Motorboating sounds like a power supply problem, and while excessive current draw shouldn't cause motorboating maybe it is the way you have this amp built. You would need to be religious about keeping the first filter cap, output tube cathode and PT secondary CT grounded away from the preamp due to the heavy ripple current you must be drawing.

I also didn't know Doug had kits, at least in the last 10 years or so since he stopped selling completed Hoffman boards.

[Colas LeGrippa]

I also didn't know Doug had kits,

I bought only the PT and OT transformers, that was 4 years ago or so.

Motorboating sounds like a power supply problem

When I hook up a .68uF cap to the cathode of the second half of the 12ax7 only, with the volume past 3/4. Otherwise, there is no  motorboating.

Is that the 125P1B that Doug sells?

I just can't remember and it is not marked on the PT either.


The chassis is pretty huge for a Champ so the preamp stuff is to the left and all the rest to the right with their own ground. There is much room ( 4 inches ) between both sections.
All I have modified including the removal of the negative feedback is increasing the reservoir cap to 23uF ( two 47uF in series ) and all the others raised to 10uF, instead of 8uF.
The last coupling cap is soldered very close to the power tube so no signal wire runs long.
Due to the very high voltage yielded by the PT, I get 250V on the preamp tube plates : should I raise the B+ rail 22K resistor in order to get 150V as indicated on the schem ?

ThanksColas

[Colas LeGrippa]

Is that the 125P1B that Doug sells?

Yes ! I found out it is the 125P1B actually.  Getting over 400V ( 5Y3 ) is way too high for a Champ. Coppola.

[Colas LeGrippa]

Other mods I have performed: 5.6k grid stopper, 1k 5w screen grid resistor

[HotBluePlates]

Is that the 125P1B that Doug sells?

Yes ! I found out it is the 125P1B actually.

Then you working the PT hard, given the 70mA rating. Strange, that should cause the voltage to drop.

Getting over 400V ( 5Y3 ) is way too high for a Champ.

The PT is rated for 320-0-320v, like the original. I don't know if it's really providing what it says, or more. With 320vac, I figure 452vdc unloaded, so the 5Y3 has to drop a lot. Seems like even a real U.S.-made 5Y3 would still provide over the claimed 350vdc on the Champ schematic.

Motorboating sounds like a power supply problem

When I hook up a .68uF cap to the cathode of the second half of the 12ax7 only, with the volume past 3/4. Otherwise, there is no  motorboating.

The chassis is pretty huge for a Champ so the preamp stuff is to the left and all the rest to the right with their own ground. There is much room ( 4 inches ) between both sections.
All I have modified including the removal of the negative feedback ...

If there is no negative feedback, the amp should be stable as a rock at all times and all volumes. So any kind of oscillation would indicate a layout or lead-length problem. Maybe due to the large space available in the chassis?

I don't know what to tell you, other than figure out why it oscillates and make sure it never does under any condition.

Due to the very high voltage yielded by the PT, I get 250V on the preamp tube plates : should I raise the B+ rail 22K resistor in order to get 150V as indicated on the schem ?

Your voltage is reasonably consistent with later Champs. If you'd like to drop the voltage to the preamp though, raising that 22k resistor will get it done.

What you could do is measure the voltage drop across that resistor now, and make a note of it. Divide that voltage by 22,000 to find the current drawn by the preamp. Figure out how much voltage has to be dropped across the 22k to get from what you have before it to what you want to have at the following B+ node. Divide that voltage drop by your calculated current, and you'll get the value of resistor needed. Round to the nearest standard value.

For reference, the 5F1 Champ says there was 295vdc before the 22k, 250vdc after it, and after passing through the plate load resistors there is 150vdc left on the plates.

[Colas LeGrippa]

[quoteThen you working the PT hard, given the 70mA rating. Strange, that should cause the voltage to drop.

][/quote]

it drops a lot, HBP , from 450V or so to 350V


I could use a 6V6 and run the PT under rating, but the voltage would be too high at 400V. An EL34 / 6L6 at 400V would draw 60-70 mA which would be perfect. The reason to choose a 6550 then ? I love them. The PT doesn't seem to suffer from extra hard work according to the tone of the amp. I will put a lower power tube and hook back the .68uF to see if the amp is more stable. I have already experienced a PT driven hard, way over it's rating that caused extremely loud howling.

Thanx bro.

Colas

[HotBluePlates]

I just realized that I forgot you'd already mentioned that earlier in the thread.

You also mentioned a rotary switch with different coupling cap values. Are the wires from that to the following tube stage long, or go near the plate output of the 2nd gain stage? Maybe that is the cause of the oscillation you heard.

If the bigger tubes do what you want and the filament winding doesn't seem overstressed (sagging below maybe 6v or so), then go for it.

[Colas LeGrippa]

Hi bro,


Filament is 6.4V

The rotary switch is a 5 position that bypasses the volume pot, from outer leg ( in ) to wiper ( out to the next 1/2 )  First position being no cap at all. So I can't see no prob. there.
Getting back to the PT, after 4 hours of intense rehearsal, it is hot, but not to the point to fry an egg. OT is almost cold ( Mercury Mag. Axiom 4.5k ), and the choke is totally cold.Overdriven with a HOT CAKE ( remember Paul Crowther, the drummer of the 70's band  ''Split Enz'' , from New-Zealand? it's him who designed that fabulous overdrive stompbox ) it 's got a killer tone. One 12ax7 and one 6550, in the simplest circuit I've ever made.

Are the wires from that to the following tube stage long, or go near the plate output of the 2nd gain stage?

Obviously ! It goes right to pin 7, which is 1/8'' from pin no 6 !!!!!!!!!!!!!!!