Problems with Hoffman stereo preamp

[Rp3703]

So here’s the deal, I have been enjoying the sound of two heads running stereo but have not been enjoying carrying two heads around to shows. So the idea popped into my head, why not build a stereo head into one box. After looking into a couple stereo power amps I realized that Marshall made the 9005 Stereo 50 watt tube amp in the early 90’s which would fit the bill perfectly and costs used way less than I could build one myself. Bought it. Now I need a stereo preamp. Low and behold, Doug Hoffman has just the thing I was looking for on his EL34World site( http://el34world.com/projects/StereoPreAmp1.htm ). Its a stereo 2203 similar circuit built into a two unit box. In the interest of saving even more space, I figure that I could cram this thing into a 1unit space instead. I spent weeks drawing and redrawing the layout for this thing and after all the parts arrived, I spent another week fabricating the box and wiring it up. The result… bad grounding. So diagnosis time. I have run the grounding scheme in two different ways and the second attempt caused the thing to oscillate. So rather than explaining what I did in words, I have attached the schematic with lines drawn to show the ground path. It only shows grounding for one of the preamps but both are wired the same back to the send filter cap. All input and output jacks are isolated. Because of this, I ran two .01u caps to ground as shown in the drawing. The chassis connection is the same one the second filter cap is connected to. Instead of the Mojo tranny Doug uses, I used a toroidal  Antek Guitar Tube Power Transformer - 50VA 280V-260V-0V & 6.3V 2A x2 AS-05T280 that has no center tap, just like the Mojo. I just taped off the ends of the 280V and extra 6.3V secondaries. Since there are two separate preamps running in the same box, I figured it is best for them to not share a common ground until they reach the power amp. In Doug’s design, he ran a bus bar on the back of the pots and used non-isolated input jacks. He also ran a ground to chassis from the inputs and the last filter caps for each preamp. The ground path back to the power amp, I’m guessing must run through the chassis since I can’t see it in the pictures. No offense to Doug but from everything I’ve read, this is not the best way to run grounds in a tube amp, which is why I changed things. This is what I get for trying to reinvent the wheel. Anyhow, I figured I would try and get some input from the experts on this before I go changing things a third time.

[EL34]

I don't know what to tell you about your grounding scheme
All I can say is my stereo preamp is dead quiet

I use mine several times a week for several years
So far, no problems at all

Internet theories are ok sometimes, but I would do what I did since it is known to work perfectly
And it's not a theory, it's a tried and true method that works 

[Rp3703]

Again, I meant no offense to you, Doug. Nor am I trying to claim that your preamp needs any improvements. I was just under the assumption that a 1 unit version of your design was not achievable without reconfiguring the grounding scheme because the pots are not stacked. Could you answer some questions I have about your design? I can see where the grounds from the preamps come from. Is the one ground wire coming from the input jacks tied to both sets of pots and input jacks? Where does the power amp get grounded? Will it make a difference if the chassis is made of steel rather than aluminum like yours is? Thanks for putting this together. I can’t wait to get this thing working.

[Willabe]

You might need a ground lift on the pre amp.

When you plug the pre amp into the power amp because both have a 3 wire power cord you get a ground loop through the guitar cords ground and the 3rd wires of the power cords.

Pre amp power cord safety ground wire > output jack ground > guitar cord ground wire > input jack ground > power amp safety ground wire > back to the pre amp power cord safety ground wire.

And round and round she goes.

[sluckey]

At some point every ground inside that preamp will be connected together, ie, bridge ground, filter cap grounds, input/output jack grounds, pot grounds, board grounds, etc. They may not physically be connected to the same screw on the chassis but there must be zero ohms resistance between all the grounds I just mentioned. Is this the case with your build?

The power amp is an external unit similar in function to what you describe. The power amp ground is probably only connected to the preamp ground through the 1/4" patch cables.

[EL34]

I am using a two prong power cord
The guitar cables provide ground paths between the pre amp and the power amp
So make sure you do not have a ground loop between your pre amp and power amp


http://el34world.com/projects/StereoPreAmp1.htm
I had to look at my build page above to remember how I ran my grounds

Looks to me like I have a central ground point on the chassis floor
There are 3 wires going to that main ground point

The two ground buses on the board go there
The ground from the pot bus going there

The only reason my grounds are like that is because I am using a rack chassis and all the panels just slide together
So I could not be sure the front panel is in good contact with the floor panel

If I had used a regular amp chassis, the two grounds from the board would just be grounded to the pot bus
That's exactly how I do all the amps

[Rp3703]

So Doug, do you remember how the power amp is wired. From the drawing it looks like the filter caps are grounded together but I can't see where the ground of the bridge rectifier goes. From the pics, the only wires I see going from the power section to the preamp's are the heaters and B+. What is the return path and how is it made.

So if everything in the preamp stages gets grounded to the pot buss', will it matter if my input jacks are isolated from the chassis? Is it better if they are not isolated? Marshall grounding is done similar to what you are describing and they use isolated input jacks. They also don't have the caps to ground to reduce RFI. Should I toss those as well?

The chassis on this thing is already built for a three prong plug. If yours is two prong, are you saying that I should completely remove the ground connection from the chassis? That sounds dangerous. Is there some sort of ground lift circuit I should use instead?

Sorry for my ignorance. I only have a basic understanding of this stuff. I can read schematics and understand layout but I know practically nothing about what's actually going on.

[EL34]

So Doug, do you remember how the power amp is wired

There is no power amp?
Do you mean the power supply?

A lug on the chassis is the ground point by the power transformer for all the grounds on that power supply board
Just like in many amps, the PT has a ground connection and connects back to the pot bus through the chassis

I don't know about the isolated jacks, I never do that
It probably does not matter because the pots are all touching the chassis and they have a bus wire down the back of them
The jack ground is connected to the pot bus

[Rp3703]

Yes, sorry. I meant the power supply. Thanks again for your time Doug and everyone else who replied. When I get a few free hours to rewire this thing, I’ll post how it turned out.

[Rp3703]

If you have time can you tell me if this layout I've drawn looks like it will work? The only two questions I have are: (1) Is it alright to connect the connect the ground from input jack 2 to input jack 1 and run it through to the pot buss or should I run a wire from input jack 2 over to the pot bus(I left that connection out of the drawing to see what you recommend)? (2) Is it alright to run the ground bus from the master pot of channel 1 over to the gain pot of channel 2 or should I run a wire from the gain pot of channel two to the chassis ground connection(I also left that connection out of the drawing to see what you recommend)? Thanks again for your time.

[Rp3703]

So after many alterations on Doug’s design, I got this thing working. I think my initial problem was that I was trying to make one channel stock Doug’s design and the other a JCM 800. I’m not positive but I think that the shared ground between the medium gain and high gain pre amps was causing my problems. After discovering that neither pre amp flavor was my thing, I decided to YouTube early Marshall models to find out what was. That is where I can across a great video by Johan Segeborn who sits in a room with 15 different early Marshall models and plays through them all. That’s how I figured out that the 1971 Marshall Super Lead was what I was looking for(actually the 77 Super Bass sounded better to me but the one he is playing out of had been modded, so who knows what was going on inside that head). So now I have a dual channel 1971 Super Lead pre amp in front of a Marshall 9005 dual 50 watt tube power amp. I slap a EHX Soul Food in front of that and it freaking roars. The best guitar amp tone I have ever heard. Thanks to Doug for posting his design.

[EL34]

Cool,
Glad you got it all sorted out and working

[sluckey]

That's a monster rig! Nice work too.

[Rp3703]

Yeah, with three transformers in the power amp and one more in the pre, this box has a bit of weight to it but it is still smaller than carrying around two heads. The economics of it is what won me over. A 1987 can run a $1000-$2500 by itself. The 9005 is essentially the classic Marshall power amp design they have used for decades with a couple of 12AX7's thrown in to run it at 25 watts if you choose to. They run around $400 used any day of the week. The dual channel pre was maybe $200 in parts. I may not have the resale value of a genuine 1987 but I'll take that deal any day.

[tubenit]

That sounds like a whopping success!  Bravo!  And thanks for sharing the results.

Glad you're getting a good tone that you love. 

With respect, Tubenit