Low Power Standel 10L12 with 6V6 amp

[kingplank]

I am planning to build a low power practice/studio amp based on the Standel 25L15. I bought a Hammond AO-29F organ amp for pennies and it is supposed to be functiional. It is now stripped and ready for the Standel conversion.
I have used a home built 5E3 for over 20 years, the 6V6's puts out just enough power for a normal drummer, so this amp will be adaquate for that. Comtemplating on reducing the out power even lower than 3-4 watts without using power attenuator. I have used triode switch on the output tudes but I read that it shortens tube life. Also read that Mesa have a switch to single ended output.
Any suggestions?

[sluckey]

I've read that the AO-29 PT may put out more voltage than needed for a low power amp. That 380-0-380 secondary on your schematic will produce an unloaded B+ of  537 volts.

[kingplank]

Very True.
The AO-29 F had a speaker field coil right after the rectifier which works like a choke. Note thatmy schematic has a R23 270ohm resistor in series with C23, 20uF ahead of a 5H choke
By varing R23 or removing C23 I can control the voltages.
The A)-29 schmatic shows 305V at the 6V6 plates

[sluckey]

The A)-29 schmatic shows 305V at the 6V6 plates

That's because the field coil is put in the center tap side of the power supply. The schematic shows 100V across the field coil. When you take the field coil out of the center tap and put the center tap directly to ground, the B+ will rise by 100V, increasing the plate voltage to 405V.

But I see you are using a choke input filter so your voltages will be considerably less than with a cap input filter if you remove C32.

[eleventeen]

Maybe you should try placing one of those two chokes in the CT of the power transformer, directly replacing the old field coil? Seems like a natural to me. You still have a choke to use "normally"..?

After you do this, if it drops the 100 v that Steve mentioned and that's more than you'd like, maybe you change to a C input filter and bring the B+ volts up some.

[kingplank]

I have simplified the power supply, and removed the input cap and the second choke

[PRR]

Missing coupling cap into grid of TP14

C21 should be other end of R50

I have no idea what D5 is for

D1 must connect to PT, *before* main rectifier

FAT pot does nothing after about 5K (since there's 2.7K in parallel), so 500K is much too much.

Merlin argues for D2 but I don't think he connects it *that* way

[kingplank]

Huge thanks to PRR for pointing out my critical mistakes

Missing coupling cap into grid of TP14

Am I locating the master volume on the wrong side of C8?

C21 should be other end of R50

My mistake here; probably explains why my other Standel built misbehaves when the volume is cranked. Better fix that asap.

I have no idea what D5 is for

Found this in newer cathode biased amps, no idea how it works either

D1 must connect to PT, *before* main rectifier

Critical mistake on my part here

FAT pot does nothing after about 5K (since there's 2.7K in parallel), so 500K is much too much

Agreed, the Fat adds a sutle change to the tone, I find it useful for playing mellow smoky jazz

Merlin argues for D2 but I don't think he connects it *that* way

Found this article http://www.valvewizard.co.uk/dccf.html
I thought the diode here my be useful

[John]

IIRC, D2 should connect directly to cathode, not after the K resistor. Also, I *think* Merlin showed a 10k ohm resistor just before D2, although I'm not sure that value is critical.

I've not done any of this, I just know what you're referencing.

[PRR]

> wrong side of C8?

You are coming out of a stage with DC on the plate.

You are going into a stage with DC on the grid. And not the same DC as the previous stage.

The pot will almost certainly be DC grounded. (There's other ways but they have problems.)

You need cap before *and* after the pot.

[kingplank]

Corrected c21 and R50 makes the amp louder and fuller, than before.

Disconnecting D2 and D 5 made no sonic difference

[sluckey]

Did you put a cap between the wiper of the master vol. and the grid of the PI yet? Really need that cap.

Also, D7 and D8 are only rated for 1000V. All the amps I've seen with those flyback diodes used a 3000V rating, or 3 series connected 1N4007s.

[kingplank]

There are times when I need full 10 watts when I play with a small jazz combo.

I have revised the schematic, added a triode -pentode switch.
The original Hammond A029 survived 50 years without flybacks; do we still need them?

[sluckey]

The original Hammond A029 survived 50 years without flybacks; do we still need them?

Right. But the AO29 was located inside a big cabinet and hard wired to a speaker and was very unlikely to be played loudly without a speaker connected. I would not bother with the diodes on a low power amp. Flybacks are usually seen on high power high voltage amps.

[Willabe]

The output signal from V5 is split at C7/C8 and goes to V4 through tone controls. I don't see where V4 output goes?


                Brad      

[sluckey]

V4 output is on the cathode.

[Willabe]

V4 output is on the cathode.

Where? To pin 3 of V3?


             Brad     

[sluckey]

Yes. At least that's what I 'think'. I don't recall ever seeing that type of feedback tone circuit.

[Willabe]

Yeah it's different alright. I missed it.

Who knows might sound good?


           Brad     

[Willabe]

Yes. At least that's what I 'think'. I don't recall ever seeing that type of feedback tone circuit.

I take it that because the tone controls are in the FB loop their active?


              Brad     

[kingplank]

Original schematic of the Standel
Chet Atkins recorded and giged with this amp for 25 years.
There are many arguements whether this is the exact schematic of Chet's.
Regardless, my previous built sounded an awful like Chet, (or Cliff Galup if I use 2 single coils at mid position). Not Fenderly at all compared to our many Fenders we have at home.

[HotBluePlates]

I take it that because the tone controls are in the FB loop their active?

Yes, because twiddling the knobs changes the gain of the stages in the loop, if you look at those stages as a black box. By comparison, normal tone controls result only in a loss of signal, and do not change the gain of the active stages they are inserted between.

There are many arguements whether this is the exact schematic of Chet's.

It's not, but you can't prove that conclusively without having Chet's amp in front of you to document. It's close enough to other 25L15 amps to make it useful, but the schematic fragment is only the preamp and the preamp portion of the power supply.

[kingplank]

Is this a better or safer way to back bias?

[PRR]

> to back bias?

Self-bias and back-bias are essentially the same when 95% of current is in the one stage.

Back-bias is useful when you have a second _large_ load, such as a radio front-end or speaker field-coil, to swamp-out variation in power-stage current.

[kingplank]

Is it a good idea to first power up using an elcheapo wall adaptor (9V DC) connected after the standby switch to check some basic connections?
Anybody tried that?
thanks

[Willabe]

No, build 1 of these and use it, it works great.

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


        Brad     

[kingplank]

Thanks Willabe, I've built one, and together with a veripot; I soft started the amp. The lamp really worked when I accidentally shorted something with my meter probe.
After fixing one non-fatal wiring mistake, the amp works!!
Very nice sounding amp. defineitely Standel-ish; a little mellower sounding than my other Standel built with 6L6.  I kinda expect that from 6V6's anyways (the OT is physically much smaller too) No thunder loud amp, just exactly the power I need for home studio use.

Nice surprice too, all 5 of the old tubes that came with chassis works, a liitle concern that my B+ is about 10V higher than the Hammond M3 schematics calls for. I want these 10 year old parts to survive.
Next task is to build a nice cabinet, seperate head and speaker.

HBP will be pleased that I had abandoned the fixed bias option.

Have not wacked the swith to tiode mode yet; wonder if my voltages are safe?

[HotBluePlates]

Have not wacked the swith to tiode mode yet; wonder if my voltages are safe?

How much voltage is across R20? What dissiaption do you have at idle now?

If R20 voltage drop is more than a couple-volts, plate current will rise significantly when you switch to triode mode.

You didn't copy the adjustable cathode bias plan I provided previously, so your tubes may well redplate in triode mode. Even if you did use adjustable cathode bias, you may not have enough adjustment available for ideal bias in both settings.

I original put a Pentode/Ultralinear/Triode switch in my 25L15; I had to yank it out after the first test because of the overheating issue I described. That switch isn't compatible with the original 25L15 power supply.

[kingplank]

How much voltage is across R20? What dissiaption do you have at idle now?
If R20 voltage drop is more than a couple-volts, plate current will rise significantly when you switch to triode mode.
[/quote]

There is a whopping 21V across R20, I have yanked out the triode switch now.

Another query: Plate voltsge on the first tube 12AX7 is only 90V. Swapping out R3 to 68K increases it to 190V.  There is slightly more gain and the amp sounds a little blusier. Is a hi-low power switch a good idea here for more tonal options?

[sluckey]

Changing the plate resistor from 270K down to 68K should decrease the gain quite a bit. If you like the sound you could leave the 270K in place and switch a 91K in parallel with the 270K. That's the equivalent to 68K and the switch 'pop' will be reduced, maybe even eliminated.

[HotBluePlates]

There is a whopping 21V across R20 ...

That's not much.

Because your power supply doesn't match the original circuit, you don't have the 100-200v difference between the plate and screen of the output tubes.

With a 100v increase in the screen voltage, you can see why my amp had immediate redplating when switched into triode mode.

... Plate voltsge on the first tube 12AX7 is only 90V. Swapping out R3 to 68K increases it to 190V.  ...

For this, use whatever gives the sound you want.

Again, because your power supply doesn't match the original, you don't have the ~320vdc at the filter cap feeding that stage, so you don't get the 110-130vdc at the input stage plate with a 270kΩ plate load.

I can't tell you whether that will make any significant change in the sound, as I haven't tinkered this circuit. Mine has stayed basically stock except for a couple of tone controls.

[kingplank]

The project is now complete, I am very pleased with the tone of this amp.
It is a little warmer and less metallic than my other Standel-ish built; less omph on the low E string, and it is very suited to the type of music that I play. Also takes stomp boxes very well.

Finally installed a 12 position switch on the T filter; cap sizes 150p to .0068u; sounds crazy, but it has become my go-to tone control.
Cabinet from off cut Finnish 14-ply

Its been a very stressful project, but highly worth it

[kingplank]

It is now 10 months since I built this amp, used daily as a practice amp. Still works like new. No issues. Those 50 year old tubes are still working.

I had finally purchased a suitable speaker , a mint condition JBL D123. This is a small version of the D130.
The sound with 2 single coils is so creamy smooth. I can never get tired with this tone.