Moody GA-40

[TIMBO]

Back again..............
I liked the COOL head cab,so i'll be making a slightly bigger cab in the same theme. Thanks

[tubenit]

That is a very interesting amp design!  I like it.

Can you share some more about the reverb circuit?  There is no reverb transformer?  What are you using for a reverb tank?  How does it sound?

With respect, Tubenit

[TIMBO]

Hi guys, Progress is moving along nicely. The PSU for these old amps always seem to be a bit light on when it comes the filtering and the number of circuits hanging off one NODE. It must have been a cost thing.

Does the PSU need more stages/filtering.Thanks

[TIMBO]

Hi guys, Some of the local blokes have been giving me a hand with the circuit and I have made some changes.

Cause this schem is a tracing from the amp itself, I can only trust that the guy has drawn it as he saw it.

  There seems to be a concern that the vibrato is connected via the two cathodes, could this be right as there is not a lot circuits to compare to. Thanks

[HotBluePlates]

There seems to be a concern that the vibrato is connected via the two cathodes, could this be right as there is not a lot circuits to compare to.

What looks wrong is that the oscillator buffer (with the 680Ω cathode resistor) has a cathode bypass cap... or even its own cathode resistor.

Lots of small amps couple the trem signal to a preamp stage cathode like this: they change the current through a shared cathode resistor, which bounces the bias of the preamp tube, which adds trem.

The 100uF/680Ω combination is -3dB at 2.3Hz, so it would seem to shunt the trem signal. Giving the buffer its own cathode resistor would seem to discourage the buffer controlling the preamp stage's current.

[TIMBO]

Hi guys, Just a couple of pics to get this started. I'm thinking that the vibrato will have to be built to see if it will work as drawn.

[lego4040]

That's some amp there, I know nothing about these amps but it's gonna be a killer

[HotBluePlates]

I'm thinking that the vibrato will have to be built to see if it will work as drawn.

That is always the best way.

[tubenit]

That is a very interesting amp design!  I like it.

Can you share some more about the reverb circuit?  There is no reverb transformer?  What are you using for a reverb tank?  How does it sound?

With respect, Tubenit

[chocopower]

I found an article were they speak about the tremolo in that amp.


http://www.ozvalveamps.org/lfofilter.htm

[kagliostro]

I think TIMBO is at home on ozvalveamps 


K

[TIMBO]

Hi guys, Not a lot of info on these old amps there is this Moody GA40 clip so the reverb will have to be built to see what its like

[sluckey]

Timbo, there seems to be an error on your vibrato oscillator circuit. You have a 1M resistor connected between V3 pin 6 and ground. I believe that resistor should be between V3 pin 7 and ground.

[TIMBO]

Thanks Sluckey, I have copied the original tracing of the amp, so if my drawing is wrong that is the reason. To me the circuit looks OK, but I am a were that there could be problems. The other catch is that the guys that designed some of these amps were a bit creative in there design and engineering. Thanks again

[sluckey]

What I'm trying to point out is an error on the sch schematic you posted. It's not part of someone's creative design. I doubt the real amp is wired as you drew it. The tube cannot work that way because there is no grid return resistor, therefore there is no bias established between grid and cathode.

Here are the circuits side by side. Hope this makes sense to you.

[TIMBO]

Thanks Sluckey, I went through some of the schematics here http://www.ozvalveamps.org/ and there is a lot of similar circuits to the attached schem. Thanks

[TIMBO]

Hi guys, Now that the mystery amp has had some of the mystery removed I have decided to change some of the output stage.
I will reuse to PT in the other build and replace it with a 50w Hammond 1750M and I have a pair of 5881s that I'm keen to use.Thanks

[jjasilli]

I found an article were they speak about the tremolo in that amp.


http://www.ozvalveamps.org/lfofilter.htm

Great article!  Thanks.

[TIMBO]

Hi guys, Finally got the wiring finished and fried it up (with out tubes). Heaters lit up OK and measured just over the 6.3v.
The HT side measured a bit higher (195v) than the specs (175v)of the PT but I assumed it would drop a bit with a load and lower wall voltage.
At 195v voltage doubler has it at 520v  way over the e-caps voltage rating so I changed them to 500v hoping with a load it would drop a bit.

With tubes in, not a good result, HT fuse blew. Checked all PSU related parts for anything obvious but nothing stood out.
Pulled tubes and one by one replaced the preamp/PI tubes and all was good.
Added the power tubes and fuse blew.
Upped the HT fuse to 630ma and still on good.
I also increased the power tubes cathode resistor to lower the output and still the fuse blew. 

 

[PRR]

> at 520v
> Hammond 1750M - 4.2K ohms
> cathode bias

This won't work. Either 6L6 current is WAY too high, or you have to strangle the 6L6es so much you won't get much power with that low impedance transformer.

350V-380V may work. 

[TIMBO]

Thanks PRR, This build is way over my pay grade. When I got the PT from the MYSTERY amp the specs had the HT at 175v but when fired up it was more like 195v this really pushed everything to the limits.
The original moody PT had the HT at 170v.

I'm getting the feeling with the old amps were pushing the boundary's and there live expectancy was short.  

[PRR]

> getting the feeling with the old amps were pushing the boundary's and there live expectancy was short.

No. You are throwing-together parts from a Chevy, a Porsche, and a John Deere. Each is a fine machine. But not made to work with each other.

[TIMBO]

Thanks PRR, I used that same transformer in another 6L6 cathode bias amp with about 390v on the plates and it sounded great.

I'm looking into another transformer that should put out a B+ of 360v. I'm hoping this will help with a few problems.

There still seems to be some doubts that the reverb resistors are correct value and the position of the on/off switch....

And the vibrato V3b cathode tied to the V2a cathode also is scratching heads. Thanks

[TIMBO]

Hi guys, The over here have been helping me with this one http://www.guitargear.net.au/discussion/index.php?topic=40603.0 and got a few problems sorted.
The HT fuse kept blowing and it took a bit of math to realise that the 500mA fuse was a bit light on, so it was upped to 1.5A and seems to be holding well.
With this sorted I got the power tubes at a good idle of 25w/tube.
Originally the 4.7k resistor between nodes A and C got cooked so V3 and 4 were removed get the amp working.
I paralleled two 10k/5w resistors to replace the 4.7k smoker
At start up it past the smoke test with a small amount of hum a buzz, Sounds great.
Added V3 and the vibrato works in a fashion. Speed is OK but the intensity get thumpy and buzzy when turned beyond 2 on the dial similar to this
  amp noises

I also getting some RICE BUBBLE effect at times. Thanks

[TIMBO]

Guys, With a bit more knowledge under my belt, it's time to move this one along.
Taking on board PRR's concerns...

> at 520v
> Hammond 1750M - 4.2K ohms
> cathode bias

This won't work. Either 6L6 current is WAY too high, or you have to strangle the 6L6es so much you won't get much power with that low impedance transformer.

350V-380V may work. 

A fixed bias is now added and I'll endeavour to get the main part of the amp working.

[trobbins]

Hmmm, you may get some interaction between sagging/recovery of the main B+, and the bias voltage, due to level shift on the 'A' supply midpoint - interesting to see how that works out.

[TIMBO]

Hey Tim, A bit more done today and this is what I have got.

I have trimmed the PSU down to supply the main amp (PA, PI and Pre)


I think the voltages look good, maybe a bit high on the 6BL8.

[TIMBO]

Hi guys, Had a bit of a play with the vibrato.
With or without the shared cathode resistor the THUMPING is present, seems to be a common problem.
So a rewire to a Fender bias vary of the power valves, problem sorted.

I have noticed a .047 coupling cap prior to the tone stack, being a basic Fender TS this cap doesn't exist.
Is there any benefit in having this cap, or could it be a mistake in the tracing.
Now to tackle the REVERB.

[sluckey]

That .047 is needed to block dc plate voltage from reaching the tone and volume controls and also the grid of the next stage.

[PRR]

That tone-stack omits the ~~0.022u cap to the top of the bass pot.

With Fender's design, the whole tonestack DC-blocks the previous plate from the next grid.

Without the omitted cap, it doesn't; so you need a cap in front.

[TIMBO]

Thanks guys, I'm slowly nutting out the problems.
PRR, that TS IS wired as per my schem, There is a few Moody amps that use the circuit and a couple of guys here have done tracing.
http://www.ozvalveamps.org/moody/ba40cct.gif
I have the reverb working and having the same problem of the dropping resistor getting stinking hot.
I'm using a 10k/10w off node "B".This gives me about 200v on the driver plate.
On other Aussie amps the 6GW8 and 6BM8 are widely used for the reverb and these also have a large wattage resistor connected to the driver plate.

[DummyLoad]

Thanks guys, I'm slowly nutting out the problems.
PRR, that TS IS wired as per my schem, There is a few Moody amps that use the circuit and a couple of guys here have done tracing.
http://www.ozvalveamps.org/moody/ba40cct.gif
I have the reverb working and having the same problem of the dropping resistor getting stinking hot.
I'm using a 10k/10w off node "B".This gives me about 200v on the driver plate.
On other Aussie amps the 6GW8 and 6BM8 are widely used for the reverb and these also have a large wattage resistor connected to the driver plate.

timbo, use a 3.9K-4.7K 10W. it'll work just as well and run somewhat cooler.


--pete

[TIMBO]

Thanks DL, A 4.7k gets me 260v on the pentode plate and a 6k about 245v
Data sheet
http://www.nj7p.org/Tubes/PDFs/Frank/049-RCA_Cuningham/6DX8.pdf
Shows a bit lower for typical voltage.
The 4.7k/5w gets pretty toasty, to be expected.
The 6k/10w also still pretty warm.
As said some of the vintage amps use a 20W resistor.
So a string of 1k/5w should get a good plate voltage for the 6DX8.
This is how it is wired at the moment, could use some tweaking????

[tubenit]

Timbo,

Thanks for sharing your progress and success with this build!  I've been following this with interest.  Can you please post what your final schematic has for the reverb?   I will then place that in our ARCHIVES  "one tube reverb" thread.   Also please indicate whether there is a reverb trannie and what you are using for the reverb tank.

Thanks!  With respect, Tubenit

[TIMBO]

Hey T, I'm lucky enough to have snagged another vintage amp that uses a very similar circuit.
Similar to this.
http://www.guitarnerd.com.au/2012/03/maton-professional-v606-amp-for-sale/
I have not given it the once over yet to check voltages etc.

A can do on the final schematic. Thanks

[TIMBO]

Hi guys, I'm not sure what's going with the reverb circuit, it seemed to be working and it started to frat out after being on for a few minutes.
Tried all sorts of things to get it working but no joy.
I have used the 6BM8 circuit and have not had any trouble with it, so a rewire and so far where good.

This is where the circuit is for now.
I think I'm still having some issues with the vibrato............

[tubenit]

Hey Timbo,

Thanks for sharing the schematic and reverb info!    I would be tempted to try the reverb as I am showing it in this schematic.  And I think you may need a cap of 500p-.001 range for going into the dwell pot.    IF you increased the reverb pot to 1MA, then I think you'd have more reverb on tap and it could help with moving the insertion point also.

Just a couple of thoughts to consider.

with respect, Tubenit

[kagliostro]

Also, 3 x 220K mix resistor intead of 2 x 220K + 1 x 270K ?


Franco

[TIMBO]

Thanks guys, The 6BM8 reverb circuit is working well with plenty BOING.
T, I rewired the input as per your schem.


With this the PSU was tweaked to get the voltage right.

As you can see I had to put a large wattage dropping resistor off "B" node (a string of 1k/5w) to cope with the amount of current.
I also increased the Plate resistor (2x10k/5w ll) this resistor gets very hot almost too hot to touch.

These voltage look good, maybe a bit high on the 6BL8????

The last thing to address is the vibrato........
The voltages (oscillator OFF) are pretty close to the Fender 6G9A and there seems to plenty of wobble.
BUT, I'm getting some static on the pulsating note, more so on bass notes.
To reduce this I turn down the volume of the guitar (humbuckers) but if you dig in it is still there.
With the bias set at around 44mAs and the oscillator on the bias spikes to about 70mAs, could this be just enough to upset the valves causing the static sound.

[TIMBO]

Hi guys, A few build pics.






With lots of long signal runs and large transformers, very very quiet.

[tubenit]

Tim,

Hey thanks for posting your success with the reverb in the ARCHIVE's "one tube reverb" thread.  Very thoughtful of you to do so!

with respect, Tubenit