3rd gen 50 Watt

[marcos]

Hi telentubes,
nice looking build. As for the cap on the Bass pot, some are ceramic as in the 2nd generation amp in the files section, others,
including mine, had orange drops. Again, this cap is only active in "Jazz" mode, so I guess it is not too important for most people (I have yet to meet a player whose favourite setting is "Jazz", although I personally think it is very usable).
As far as cap orientation is concerned, HAD obviously did not pay attention to this on the 2nd /3 rd generation amps, all the real amps and pics I have seen show the caps with the lettering facing the left side of the chassis,
despite the markings on the caps. So if you wanted to build a period correct amp, you´d have to put in most caps the "wrong" way. I would not recommend this, just sayin. As a matter of fact, I changed the cap orientation on a couple of caps in my amp, and felt the sound improved a little, but I did not notice a difference as fas as noise is concerned.

Marcos

[mlp-mx6]

I have a rotary switch coming for speaker ohm setting, but I may not have allowed enough room for the switch, in which case I'll wire the jacks either for 4/8 ohms, or 8/16 ohms. Anyone have an idea which would be the most useful?

They make SP3T toggle switches. I have used the Philmore 30-135 SP3T switch before with good success.

http://www.radioequip.com/servlet/the-1 ... dle/Detail

[martin manning]

If you have diodes in series it is a good idea to parallel a cap with each one to equalize the voltages. It's the same principle as the balancing resistors on the series connected reservoir caps. Nice looking build, there!

Hi Martin, If Im not mistaken, since the Vf is so low ~.7V, it's not a problem in the forward direction, and if using the 1N4007s, they have a Vbr of 1kV each. Aren't the caps for smoothing any spikes at the knee of the Vf, which is why FREDS are preferred?

It is in the reverse-bias direction where the balancing caps are useful, to truely get 2x or 3x the 1kV of a single rectifier when they are doubled or tripled-up. Re FREDS, I'm not an expert, but I believe they are all about the switching speed, to avoid conduction in the reverse direction at high frequencies, especially in switch-mode supplies. The 60 Hz in a FW or FWB supply is no problem for ordinary rectifiers. I have seen claims that the power supply noise is reduced with fast rectifiers, but I don't know if there is anything to that. I would think that a gentler switching characteristic might actually be better. It would be interesting to get some discussion on this. I made a couple of pics of a 1N4007 at 100 Hz and 10 kHz here: https://tubeamparchive.com/viewtopic.ph ... 281#187281

[BobW]

If you have diodes in series it is a good idea to parallel a cap with each one to equalize the voltages. It's the same principle as the balancing resistors on the series connected reservoir caps. Nice looking build, there!

Hi Martin, If Im not mistaken, since the Vf is so low ~.7V, it's not a problem in the forward direction, and if using the 1N4007s, they have a Vbr of 1kV each. Aren't the caps for smoothing any spikes at the knee of the Vf, which is why FREDS are preferred?

It is in the reverse-bias direction where the balancing caps are useful, to truely get 2x or 3x the 1kV of a single rectifier when they are doubled or tripled-up. Re FREDS, I'm not an expert, but I believe they are all about the switching speed, to avoid conduction in the reverse direction at high frequencies, especially in switch-mode supplies. The 60 Hz in a FW or FWB supply is no problem for ordinary rectifiers. I have seen claims that the power supply noise is reduced with fast rectifiers, but I don't know if there is anything to that. I would think that a gentler switching characteristic might actually be better. It would be interesting to get some discussion on this. I made a couple of pics of a 1N4007 at 100 Hz and 10 kHz here: https://tubeamparchive.com/viewtopic.ph ... 281#187281

Martin, my background is in SMPS design and I use FREDS for the same reason. I agree w/ what you've said and although most of my switchmode applications are in a greater than 100kHz, just thought if they work well in those frequencies, then they should work well in guitar amps. btw, a nice discussion, and pics of the 1N4007

[telentubes]

mlp-mx6,
The toggle looks like a good option. I could mount it "underneath" (not on the back panel) so it would be harder to switch it accidentally.

[telentubes]

Some progress. Getting there slowly.

[Max]

The coax is stunning RG188 from Apex Junior. It's silver plated copper...

Why did you decide against RG59/U foam?

Cheers,

Max

[telentubes]

Hi Max,
Mostly because the foam is hard to work with. I tried to read as many post as I could about the use of coax and came to the conclusion that it might not make that big a difference. Some folks said it would make no difference. Others said it is very important to "the sound" (of the earlier amps). This could be a case of "hearing what I want to hear" because I think the foam is kind of unruly, but as I mentioned, I can change it out if it's not appropriate.

[Max]

Mostly because the foam is hard to work with.

I see - didn't know. Thanks for the info.

Cheers,

Max

[telentubes]

Fired it up last night. Clean channel tones are lovely. I was hoping not to fall into the "couldn't figure out the relay" camp, but I did (this is my first D style amp and first relay), so I don't know what the OD is going to be like. I'll jump back into that when I have time. I also didn't hitch up the FET yet as I wanted to go through the adjustment procedure that M M had described. I took voltage readings (to post later), and adjusted the bias to -36 for starters. Adjusted the PI trim pot to around 6 Ohms difference, again as a starter point. I'm plugged into a Vin 30, running JJ "start up" tubes and will just let it cook for a while while I do other stuff. The amp is dead quiet at idle in "clean" channel.

[telentubes]

Voltage chart
B+5 306V
B+4 310V
B+3 421V
B+2 443V
B+1 445V

V1/pin 1 196V
V1/pin 6 197V
V2/pin 1 203V
V2/pin 6 201V
V3/pin 1 280V
V3/pin 6 271V

V4 and V5/pin 3 445

[martin manning]

Typo on V3 pin 6? Otherwise it looks pretty good. If you don't have the FET 150k and ~25K connected to B+5, that will lower the preamp voltages a bit and it should fall into line.

[telentubes]

I figured out the relay. I had two of the wires reversed on the relay board. Now the OD works, but needs some tweaking as the tone doesn't rock my world as it is. Love the clean channel though.

Also saw a possible discrepancy from layout to schematic on the "manual" switch. I wired the switch as per the schematic, with the .01 cap connected to ground (top lug) and the other end to the center lug. The layout (from "3rd Generation Period Correct ODS 50W") has the .01 cap connected to the two outside lugs, which I think is incorrect. Any one else notice this, or will it work both ways?

I'll have to re-take the photos I had planned on posting as I've corrected some wiring since I took the pics.

[martin manning]

I agree with you on the cap wiring- common lug to ground lug on the switch, and it does not work both ways.

[telentubes]

Martin,
The voltages on V3 were a typo as you pointed out. I made the correction. Thanks for pointing that out.

I'll mention the "manual" switch .01 cap discrepancy in the Dumble Files section as well. I will also make a layout drawing for folks who will be using the R40-11D2-12 type of relay. Might save a newcomer like me some time.