New Build- Intro to Amp Design

[SilverFox]

So I've got the parts together and some basic schematics to start working with. I'll get the ball rolling by posting the preliminary schematics and pics of the hardware. I've got to merge these two schematic together or start with something different if they won't work.

I want to use only the PT from the Mcgohan circuit but posted the entire supply to give an indication of the previous application. The filter section will have to be redesigned and likely consist of a 40uf reserve capacitor with the remaining supply caps being, 20uf.

The preamp tubes will be AX7 with typical plate voltages common to a Clean to Crunchy gain level. I get my High gain sounds from effects.

This supply previously powered a 35 watt amp. I'm going to stick with that power level. I'm also going to use the existing ground lift for the filaments unless advised otherwise. B+ will also remain the same. From there however I don't know what the voltage levels should be at the various points in the driver circuit.

The PA design is from the Univox 1221 and the PS circuit there employed a voltage doubler that ran the output tubes at a little over 600 VDC.

I will come up with a list of questions in an effort to finalize the power supply design such as Choke or Resistor input and the benefit of adding capacitors to the rectifier diodes; Best practice for fusing to protect the OT etc.

Edit- And the practicality of using a Morph control on the Pentode as a possible MV. This was suggested as the outcome when used in this stage by: Martin Manning- "Adding a "morph" control would be an interesting experiment. It would lower the gain and act as a power reduction or MV. What effect it would have tone wise, I can't say.
"

Feel free to post alternate PS schematics that would work with this PT.

The preamp section will likely use 3 valves and I would like to have the option of adding perhaps 4 other valves for either an effects loop or an Overdrive. This power transformer should support 5 amps of filament current. If not I'll use a different PT from a Magnavox Console Stereo. I think there were 14 valves in the totality of that circuit and 30 watts of output power.

If there is any interest I wouldn't mind approaching this build as a basic Introduction to Amp Design Using Existing Circuits" and structuring the posts for the benefit of others attempting to kludge an amp from various circuits.

Thanks for any help the Forum can give on this project,

Silverfox.

[tele_player]

Given that PT, why not just do a Fender 2x6L6 power amp?

Robert

[SilverFox]

I used the Univox for a while and liked the basic sound with an effects box. It needed a complete recap and I wasn't too keen on that since the sound was okay but not what I wanted.

I also want to experiment with the 6AN8 and Cathodyne? PA topology as well as try the Morph control on the pentode. I can't say for sure but it seemed as though the Uni had some very nice harmonics to the basic tone.

I've built a Fender style 6L6 power amp and if the Cathodyne doesn't sound right I'll switch back to that with a 12AT7 as the pi. Sounds very good. I've got time, curiosity, parts and the Rainy season here has begun.

Silverfox.

[SilverFox]

Regarding the operating voltages:

The original design of the Univox 1221 is based on 600+ volts supply.

If I lower the B+ to 430 VDC, ( aprox 1/3 ), and build the driver and power amp section according to the schematic per the original Univox 1221 output section, can I expect the reduced voltages across the 6AN8 valves to result in a functioning circuit or does this need to be redesigned?

Why? As much as possible I'd like to construct the PA as the original and operate at lower voltage.

Silverfox.

[martin manning]

Notice the Univox has a doubler supply, with the plates at 660V and the screens and the rest of the preamp running from the 320V at the middle of the stacked reservoir. For your amp, build a conventional FW or FWB supply at 430V for the plates, followed by a choke and a filter (L-C) for the screen node, then another R-C to get you to down to the ~300V for the 6AN8's, etc. Maybe play with the Duncan PSU simulator?

[SilverFox]

Thanks,

That will give me a starting point. Wasn't sure if I should still try to run the driver tubes at the same voltage level or that would have been too high.

I should be able to start the PS tomorrow.

Silverfox.

[SilverFox]

I don't use Windows so Duncans PS calculator is of no benefit to me. So in searching I found this on another forum while trying to determine the best Choke value to use in the power supply:

To calculate a minimum inductance value for the choke, the number 200 is key !! Divide 200 by the sum of the two capacitors before and after the choke (resistor) to give the minimum choke inductance value in Henries.

If not, how do I determine the best value for Choke in the filter circuit?


I'm also curious what value ground lift voltage to use. I'm thinking I want about 25 or so VDC ground lift on the CT of the filament windings

Another question I have, ( I used to remember this but that was last amp build season), Should I ground the unused CT on the bias winding or let it float? I'm thinking it should float if I use a bridge rectifier circuit in the bias...

Best regards all,

Silverfox.

[martin manning]

Don't ground the CT if you put a FWB rectifier on the bias winding. It is sitting at about 1/2 B+.

The filament circuit ground lift voltage isn't too critical. I'd go 50V or so. I guess you're interested in keeping the h-k voltage on the cathodyne inverter at a reasonable level, and if you have a cathode follower in the preamp it will help there too.

No reason not to use a cheap and readily available 4H choke ala Fender. That will be enough to smooth the PS ripple. More than that and it will be about a difference in feel when playing.

[SilverFox]

Very much started.

The mixed colours on the PT result from adding extensions through several other builds in an effort not to eat up the original tap leads.

I do wonder if the Choke is properly oriented with the PT. I know if there is hum the headphone trick may be used.

The planes of the fields are not aligned so I don't think I'll have a noise issue.

Any thoughts?

Silverfox.

[xtian]

Your choke won't introduce any hum regardless of its proximity to the PT.

What the choke CAN do that I found interesting, is deflect the magnetic field. I had my PT and OT laid out about 4" apart, using headphone trick. Then, I placed the choke between, and found I could affect the amount of hum induced in the OT by positioning the choke (mind you, the choke was not connected to anything!).

[SilverFox]

After reviewing tones and schematics I think I'm moving to a slightly different design. It is very similar; even to the point of making me wonder if one copied the other and I don't mean Weber. Rather- Sunn vs. Univox. Here are the two examples cited for the discussion as I want to build a composite of the two schematics. I like the introduction of the TS and the information on both is more complete than the Univox schematic. Another benefit is the very low end range available from the Sunn design. Those are the reasons for the change.

Questions:

1. I want to use 6L6 tubes, (seems to me the KT66 is compatible with 6L6). Will they work as is with only a bias adjustment? Or do I change the Grid resistor to 5.7K for the 6L6 design. My plate and screens are going to run between 430 and 450 VDC and I'm using 35 watt iron. (I've got to read up on grid resistor values I guess, (got Merlins book)).

2. Can I simply employ the 6AN8 design and wire the AX7 portion in where the dual AT7's are, in Webers Design?

Are the bias and coupling values for the EF86-AT7 close enough to the 6AN8 to make it simply a swap?

If there is really a benefit to using the EF86 and AT7 in place of the 6AN8 I could just build as is and drop one of the AT7 tubes. I think this is a super clean design and it sounds that way.

Thanks for any help on this build,

Silverfox.

[SilverFox]

I've finished the amp, see photos. Ended up building a Sunn version of the power amp. No pre yet. So far it sounds very good. There are some problems I'm homing in on and it is playable even with these issues.

After going through tubes and other diagnostics it appears I have a bad tube socket. If I push on the tube it cuts out. It isn't the tube since I've swapped 3 sets in.

The bigger problem I have and need some assistance with relates to the Boost circuit- FB

When engaged I get an oscillation-buzzing sound. Not able to run with the Boost engaged. If anyone has some suggestions it would certainly help.

Regards,

silverfox

[martin manning]

Have you tried reversing the OT primary leads? You may have connected them for positive feedback.

[SilverFox]

Now that you mention it-

That would be possible. The boost switch cuts or engages the FB and I can't operate the amp with Boost on.

I'll try that- Thanks!

silverfox.

[tictac]

If the OT were reversed the amp would oscillate as soon as you power on...

If the amp works without the boost on, then the boost circuit or lead dress is likely the problem...

TT