Gibson GA-40 - How dirty are they really?

[Meddon]

Hey all!

I just finished a Gibson GA-40 inspired project (5879 channel one, EF86 channel two, 5E9A driver tremolo instead of the 6SQ7 channel two-only tremolo). This is a really fat sounding amp; even with a skinny telecaster it overdrives at low volumes. I don´t think it´s possible to play it clean at all. I don´t mind that at all, it´s a juicy, joyful and slightly compressed overdrive which sounds quite nice, but I´m curious if this circuit was really this dirty originally? Does anyone have an original GA-40 or similar Gibson that behaves as dirty as my homebrew?


Thanks!

[soundguruman]

A totally clean amp, unless you crank it to ten, in stock form.

[Chuck H]

Right. With typical humbuckers you can stay clean to about six on the volume control with the stock amp. There may be circuit differences in your amp that reduce singnal loading, etc. And this is why your amp has more gain. Without a schematic it's impossible to say for sure.

[soundguruman]

It's the mojo that's making it sound better.

[loudthud]

How do the voltages around those pentodes and the B+ compare to the schematic?

I found this schematic: http://www.el34world.com/charts/Sche...0_55_Later.pdf

What transformers did you use?

[Meddon]

Loudthud, that´s the schematic I used.
The voltages around the 5879 preamp tube are not too far from those listed on the schematic, in my amp I have 162v on pin 8, 69v on pin 7 and 3.35 on pin 3. The schematic lists 175v, 95v and 4v. The EF86 has much lower voltages; 69v on pin6, 37v on pin 1 and 1.4v on pin 3.
B+ after the choke is 337v, the transformers are for Fender Deluxe. The PT is Webers 025130, the OT is Heyboer.

[g1]

Loudthud, that´s the schematic I used. ... The schematic lists 175v, 95v and 4v.

Have another look at the schematic loudthud posted, those voltages aren't on it. Can you link to the schematic you used?

[loudthud]

Those aren't the voltages I see on the schematic I linked but, there is a note saying the voltages were taken with a 20,000 ohms/volt meter. No mention of what range was used (that changes the load of the meter) but actual voltages may be higher or lower. The way the cathode of the 5879 is connected to the 6SQ7 means the voltages will be different with the 6SQ7 removed.

Pentodes are always hard to bias because the screen voltage interacts with the cathode bias. Without a scope I would just stick a trim pot in place of the cathode resistor and adjust for the loudest clean sound. Data in old RCA tube manuals suggest that the output voltage and gain are such that there is not much headroom on the input of these tubes. A hot guitar could overdrive them before you have a chance to turn the gain down at the volume control.

[Meddon]

Here the schem I used, I quoted the voltages for channel 1 (non-trem channel) in the chart, but I must have misread them.
I'm gonna put a trimpot in place of the cathode resistor and see if I can dial in a cleaner sound.
I read somewhere on the internet that it is impossible to get a clean sound from a Ga-40, I think someone was quoting Gerald Weber, but I can't reference that.
Anyway, the 5879 pentaode and the supposed singing overdrive sound it produces was the reason I built this amp, I just wasn't expecting this amount of dirt!
With a P90-equipped SG Jr. the amp overdrives really easy, and is lovely for playing slide

[pdf64]

A bit off the original query, but is your choke rated for the full B+ current (normally they just deal with the screen & preamp supply)?
Also have you checked the 6V6 plate dissipation? A 200 ohm cathode resistor is rather low, unless the plate voltage is a lot lower than usual; with a VB+ of 337, you may be over 100% of the spec.
If you turn the channel volume down really low, can you hear any overdrive, ie 1st stage preamp pentode distortion?
Pete.

[Meddon]

The choke is Weber 022699 which rated for 9H@120ma. Is that a bit underrated?
I haven´t checked the plate dissipation of the powertubes yet. It has a 250 ohm cathode resistor.
With a P90 pickup the amp is only slightly distorted at low volume, but the overdrive is there all the time.

I´ll have the time to tweak the circuit in a weeks time, I´ll post the results.

[imaradiostar]

Not sure about the 5879 side but the EF86 side should overdrive pretty easily. I have a similar amp with 7591's and anywhere above about 1/3 volume it's dirty with P90's and it has a basic tone stack and a long tailed phase inverter! Of course...I've not played an original so I don't know.

What tube is the phase inverter?

jamie