3rd gen 50 Watt

[telentubes]

Still plugging away. Here are some progress shots. The head cab is waiting for tolex, or some kind of covering.

I've been playing the amp for about 5 days, and hearing it change and smooth out as it breaks in. I'm also getting used to the interactiveness of the pots and switches. Bit of a learning curve.

I did some tweaking along the way... snubbers on V2a and b, 47pf to ground on grid of PI, and a 6.8K resistor in place of the 10K to ground on the bass pot. I was loving the clean tone but the OD was really bassy and ragged, so I made those changes. However, the tweaks took a lot of the sparkle, openness, and soul out of the amp, so I changed everything back to stock, except for the one 500pf snubber on V2b. That did the trick. Now I can't stop playing the thing.

[telentubes]

Some of my voltages took a dive when I hooked up the FET board. Here is what I have now...

Bias -36
V1/pin 1, 171
V1/pin 6, 173
V2/pin 1, 179
V2/pin 6, 178
V3/pin 1, 280
V3/pin 6, 275
V4 and V5, 447

B+1, 451
B+2, 449
B+3, 425
B+4, 276
B+5, 268

I'm running JJ Gold pins in V1 and V2, a Chinese 12AT7 in V3 (it sounded better than the NOS), and Winged C 6L6s.
I haven't hooked up the input to the FET yet, or some sort of Ohm selector for the speaker out. I'm using a Vin30 for now but have an EL12L in storage, and a G1265 on the way. That should be enough for a head ache.

The face plate is a chunk of curly maple that I'll lacquer when I can get around to it. It will be amber or amber brown in color but won't have any labels. Just plain.

What do you guys think about the low plate voltage on V1 and V2?

Once again, deep gratitude to everyone for all the help! Couldna done it with out it.

[martin manning]

I would say get the V1 plates up a little closer to ~180. What voltage do you have at the top of the FET's 10k drain resistor? If it is not ~20V then adjust the divider on the FET board, and get the source resistor dialed in so you have ~10V at the FET's drian. That will firm up the current draw (and therefore the voltage drop) the FET creates. If you're still not happy then you could reduce the dropping string resistor going to B+3. DIfferent preamp tubes can push the voltages around +/- 5V or so.

[Structo]

Do as Martin suggests.

If V1 & V2 are still low, change the 27K dropper to 22K.

[telentubes]

Thanks guys. I'll keep chipping away.

[telentubes]

Finally got my mind wrapped around the concept of the FET board and started to mess with the values a bit. It's been a pain working on it while still in the amp, so I will take it out and use the method Martin suggested with the two 9V batteries to dial in the values.

Changing the values in the FET didn't seem to affect the B+5 hardly at all so I will now work with the 27K dropper, as Structo suggested, to bring up the B+.
I must admit, I'm a little weary of changing ANYTHING as I am so happy with the tone of the amp the way it is. Let sleeping dogs lie.

Would it be O.K. to live with the lower voltages I have on V1 and V2, or would I be missing out on something the amp has to offer? What may I expect if I make the changes to increase the plate voltages on V1 and V2?
I suppose I should try it and see for myself. I can always change it back. Any hints would be appreciated though. Will voltages affect tube life and things of this nature? Any other considerations?

Thanks for all the help.

[Structo]

Hey, if you like how it sounds don't change a thing!

You know, if it ain't broke..........

[Cliff Schecht]

It's easy enough to tack in a 100k resistor in parallel with the 27k resistor to get around 22k (well, 21.3k but close enough). A 150k resistor would bring you to 22.9k so anything between 100k-150k will ballpark the value you are looking for. This way if you don't like it, you pull out the tacked in resistor. If you do like it, you solder that fucker in permanently.

[telentubes]

The 100K brought em up (I only had a 100K and 220K with a 2 or 3 Watt rating). The 100K was close enough, although it could be a little higher. I'll get some other values in my neck parts order then fine tune from there. Now I have to decide which voltage I prefer.

[telentubes]

Even though I STILL haven't hooked up the FET, I've made a few adjustments, and finished the head cab.

Changed the 27K dropping resistor to 22K resulting in a new voltage chart...
B+
1.) 444
2.) 441
3.) 418
4.) 294
5.) 285

V1, GE long grey plate 12AX7
pin 1, 191
pin 6, 187

V2, 7025 with no brand name (I replaced a new JJ 803S which had readings about 10V higher)
pin 1, 199
pin 6, 193

V3, Sylvania JAN 12AT7WA
pin 1, 280
pin 6, 275

V4 and V5 plate, 438

I changed the bias pot tail resistor from 27K to 18K which brought down the usable range for bias settings and raised the usable range for the cathode current (I think I'm stating that correctly, the measurement across the 1 Ohm resistor on the 6L6 cathode). Anyhow, I now have these readings which I will mess around with and tune by ear and feel...
bias, -33
cathode current, V4, 36.8 mA, and V5, 34mA

I'm using a matched set of "Winged C" 6L6s that I have been saving for a long time (bought new years ago but never used). Looks like one is beginning to red plate after only 25 hours or so of playing. Kinda disappointing. I'll switch tubes an see if there is any change.

Since the "bright switch" is a DPDT, I wired it with the choice of either a 500pf or 330pf bright cap (having no cap seemed unusable).

I'm still toying with snubbers on V2. I have had none, one, and two. I'll end up with something, but not sure which configuration or values yet.

Lastly (for now), I added the "treble bleed" mod on the OD level pot, which is just cracked on. It helps to match the OD tone to the clean channel tone so that going from "clean" to "OD" feels more seamless. Fattens it up a bit but without that treble spike. It's a 500K pot and a .001 cap to ground.

I used a toggle switch for the 4/8/16 Ohm speaker out selector.

The paint job is boarder line "over the top", but actually looks better in the dimly lit settings musicians prefer. I used Tolex and grill cloth that were left over from other projects. The green Tolex will stay, but a darker grill cloth (like black) would probably look better with my garish sunburst.

There is a 1X12 speaker cab (dimensions thanks to Taylor Cox) which just needs to be covered. It is housing a G1265.

[martin manning]

Love the faceplate! Re the grille cloth I think black Vox cloth would look pretty sweet- it's got green stitching that'd pick up the green tolex nicely.

[jelle]

Looks great!

[marcos]

I recently found some notes taken while inspecting a real 3rd generation ODS a long time ago, the B+ for V1 was 260 V ( did not measure the actual plate voltage) so I would expect plate voltage to be a little lower than on the later ODS amps. Hope this helps a bit

Marcos

[telentubes]

Marcos,
Initially my B+5 was 263V, and V1 was around 170V. the amp sounded great there, with a nice looseness about it. Just thought I'd try the higher voltages that were recommended by a few folks.

One thing I'm curious about is raising the cathode caps on V1 and V2 to at least 10uf. This is discussed a bunch in some "grailtone" threads. Good reading at any rate, https://tubeamparchive.com/viewtopic.ph ... rly++grail,
and one of the links from that thread... https://tubeamparchive.com/viewtopic.php?t=3615

Anyone tried the 10uf to 25uf values and liked the results?

I'm also interested in trying "high plate" for just the OD. Anyone done this to one of theses 50 Watts?

As an aside, I just found these 25uf/25V Spragues on sale for $1.70 at Newark...
http://www.newark.com/jsp/search/produc ... sku=18F852

[Teleguy61]

tt-
Nice project, great thread.
Thanks-
George