Paralleled SE, cathode biased questions.

[Colossal]

Also, a very small observation, but I would consider mounting that 33k input grid resistor directly on pin 2 of the V1 tube socket, not back on the input jack as shown on the layout. That will block RF rather than having that length of wire to act as an antenna from the input jack to the socket pin, after the grid resistor.

[alvarezh]

@ Firestorm:

Thank you for your comments. Will further study your suggestions ( I can't make electronic evaluations as fast as you!). Intentionally, I didn't want to deviate too far off on the PS from the original Garnet G250TR schematic, for that meant I had to recalculate the dropping resistor values (I am a learning amp builder!). Will fix the missed connection to the caps from the balancing resistors (good catch!). I missed to show the PS dropping resistors values. Will update the drawing to V3.

Regarding the choke, the original design calls for a 2.5 Henry choke, the 194G was the closest I could find at 3 H. although it's an overkill current wise. Here is the Hammond choke page for guitar amps:

http://www.hammondmfg.com/guitarLineCHK.htm

@Dave:

You are suggesting to mount the grid resistor like the Dumble's. It's better, I was just trying to save on a terminal strip. Will be using a coax from jack to tube anyways, never had any RF interference by using this method although the resistor to coax junction is hanging in the air with no support, which is not the best way by far. Will use the jack-cable-resistor-terminal strip-to grid method in the final amp. Thanks.

Anyone checked against the schematic to confirm if the tone stack and the EFX out/in circuits are properly connected on the layout?

Again, grateful for your help.

[Firestorm]

Check Mouser. They show the Hammond 156M (3H, 100mA) in stock.

[alvarezh]

Thank you Fire. The Hammond specs for that series says they are tested up to 1500 volts, but the specs for the 156M rates it at 400v.

http://www.hammondmfg.com/153.htm

Do you think it can handle the 500 volts it will be operating under?

Thanks.

[Firestorm]

Check Mouser. They show the Hammond 156M (3H, 100mA) in stock.

[alvarezh]

Thank you Fire, since you insist, 156M will be, I shall comply!

All the best.

[Firestorm]

Thank you Fire, since you insist, 156M will be, I shall comply!

All the best.

That was a smartphone-induced double post. You're dropping your screen and preamp voltages aren't you? How much is the choke going to see?

[Firestorm]

Thank you Fire, since you insist, 156M will be, I shall comply!

All the best.

That was a smartphone-induced double post. You're dropping your screen and preamp voltages aren't you? How much is the choke going to see?

I just looked at your layout. Maybe knock the voltage down before the choke. Seems crazy to have to use a medium-current choke for a relatively low-current application.

But it looks like Digikey has the 159T in stock if you need to go that way.

[alvarezh]

I concur with you, but better safe than sorry. 159T then.

Much grateful for your continued support.

[martin manning]

Horacio, what's up with the 100k connecting the 1k5 cathode resistor to ground on the stage driving the KT120? That will mute the whole thing.

[alvarezh]

Hi Martin, yes, my bad, thank you for catching that. Enclosed is the original Garnet schematic, when I deleted the PI in order to convert it to a SE operation I missed deleting that 100K resistor.

Will modify the schematic and the layout right away.

Is the tone stack on the layout looking good to you?

Thanks again.

[alvarezh]

Layout updated to rev. 5.

Modified the entrance to the bass pot using the attached schematic/layout found on another thread here in the forum as reference.

[alvarezh]

Construction update:

Filter and main board images uploaded.

The holes marked in red are for the stand offs. The plate supply lines run underneath the main board. The blue 10K 3 watt is the NFB resistor, is mounted above the board to ease replacing it after listening test. Silver mica caps, Cornell Dubilier on the cathodes, Mallory 150s and KOA Speer 1/2 watt resistors. Can install MFs on the plates but will wait for a listening test first, if quiet, I'll keep the KOAs.

The KT120's cathode resistor will be installed on the chassis with a pair of terminal strips as I ran out of space to place it on the main board as originally planed.

The filter board has UF4007 diodes with F & T caps. The 220K 1W balancing resistors are underneath the filters as the leads were too short to place them around the caps. The filters are flying about 1/8" above the resistors to minimize heat transfer.

Have all the parts sans the O.T. which should arrive by Friday. Therefore, can take my time to drill all the holes on the chassis and do the build up. By the time I get the O.T. I should have fired it up without tubes, so since the O.T. should be the last part to install, I expect to be making noise about an hour after O.T. arrival.

That big KT120 will look impressive on a SE guitar amp!

Cheers.

[alvarezh]

Build update.

I have not kept my promise and I am a few days late but I had a few more important things to do. Anyhow here is the report.

The amp has been built (images attached). No smoke. Upon fire-up I could hear my signal generator but the amp sounded horrible, found the problem quickly (misplaced wire). I get a strong signal but there is some hum with and without the signal generator plugged (not a loud one) so I have to work on that. Both volumes and the tone stack controls work as expected.

Plate voltages on the preamp need to be adjusted. I have some voltage imbalances between the A and B plates within a couple of the tubes which need to be looked at. Perhaps they are caused by the difference in cathode resistor values as all the plate resistors are 100ks (will try different tubes also). V2 the 12AT7 reverb driver is running at 434 v. on the plates which according to the Twin Reverb schematic is pretty close (440 v.).

The KT120 is getting 498 v. at the plate (474 v. if measured between plate and cathode), 375 v. at the screen and 25 v. on the cathode. With a 220 ohm cathode resistor I am getting 117 mA consumption (direct reading through a 1 Ohm resistor). Plate dissipation using the Weber bias calculator is giving me 51 Watts, 60 Watts is max as per the spec sheet.

Have only tested with the signal generator at very low volume levels (late at night). Have not tested the reverb.

Have added a presence control which I will illustrate on the final schematic.

Cheers!

[martin manning]

Looks good Horacio! That is a magnum-sized bottle eh?