ODS Inspired Build; #102 Values with Buffered FX Loop

[timrobertson100]

Great looking build Glenn

The OT phase test info would be a good addition for the FAQ section too

[GAStan]

Very interesting build Glenn! Impressive figuring it all out like you did. Way above my cranium, lol.

Thank you Raoul. It's been a challenge for sure, but one I've enjoyed. I've enjoyed watching your builds, please keep posting them.

Great looking build Glenn

Thank you Tim.

I haven't spent any time with this lately but hope to this weekend. I need to post some voltages and get some opinions about them.

[GAStan]

Voltages:
B+ 450; B+2 448
PTs: 33.5/37.8 & 32.3/37.7mA
~17 watts/56%
Sovtek 6L6WXT+

B+3 430
PI 286/29

B+3a 387
FX2 251
B+3b 383
FX1 383/103 (Cathode Follower)

B+4 317
OD1 192
OD2 201

B+5 310
CL1 190
CL2 193

Heaters 7.0vac (a bit high)
Input 123 vac

[GAStan]

I put in some JJ KT66 power tubes I have to see what it would do for the heater voltage. They brought it down to 6.9vac. They sound great in this amp. Good highs. Brite. I see how they could cause the Ice Pick effect but luckily it's not bad in this amp and can be dialed down. The lows are nice and solid. Mids are good. I can see why these would be suitable for HiFi use.

FX loop has 60hz on it. Could just be the tube but first I need to address the CF plate voltage. It's at 383vdc while Martin's values I used have it at 260. I put in the wrong power filter dropping resistor, 2k2 installed should be 100k. The plate voltage on the other tube needs to come down a bit too, it's currently at 251. I'm going to change this before powering it up again.

Dumbleator_Update_MPM.png

[martin manning]

For the built-in D'lator I believe the best plan is making the D'lator signal grounds at the loop jacks. Do you have a branch off the screen filter for the loop power? That should be pretty clean; the negative leads of the extra filters can be grounded with the screen filter.

[GAStan]

Grounding as follows:

B+1 ground to chassis near rectifier/bias board with HV center tap, and rectifier/bias.

B+2 & 2b to chassis near power tubes.

B+ 3, 3a, 3b, 4 and 5 (on filter board these are all tied together) to chassis near input jack with CL, OD, FX and PI grounds. Input and FX jacks isolated and grounded with these too. FET also grounded here.

I'm certain the hum in the FX is the tube. CL had the hum, swapping CL and FX the hum followed the tube. It isn't horrible but definitely there. I'm sure the voltages aren't helping, cathode on CF is about 100vdc.

The FX power comes off B+3, PI filter section. Currently B+3>22k>B+3a>2k2>B+3b. I need to increase the 22k a bit and 2k2 a bunch. I'll got some values from 22k to 33k to try, then 2k2 up *to 100k* for the other one. Sound like a good plan?

*edit*

[GAStan]

Quick update.

Changed power supply resistors to Dumbleator tube and voltages fell into line. This helped the hum but changing the tube out helped a bunch more. Turns out I had a large plate in it, a lower gain tube really helped. I think putting it in a cabinet will help a lot more.

The Dumbleator is working well. Like a different OD section if used by itself, so far I haven't actually plugged any pedals into it. Bypass works well, in bypass it's like it isn't there-as it should be.

I'm considering the idea of making the Dumbleator footswitchable. I'd have to free up a switch, probably put the PAB and OD on the same switch? PAB, OD & FET are what I have on the footswitch now.

I've been sidetracked so haven't spent much time on it lately. Too many danged squirrels running around...

[GAStan]

Been doing some work with this amp today. A while back I robbed the PT from it to replace the one in my SSS. I just received and installed the replacement from Amp Parts Direct. A Heyboer for a Fender 100Watt, but with 320-0-320 vs the 340-0-340 Heyboer from Mojotone that I removed. New voltages:

102 w/Dumbleator Voltages
B+: 414: 36mA bias
B+2: 413
B+3a: 363; 240
B+3b: 262
B+4 (PI): 400; 268; 276
B+5 (V2): 294; 183; 180
B+6 (V1): 285; 178; 178

The B+ and B+2 are down over 30 volts.
B+3a/b are just where I want them.
B+4 PI should be 280/290
V2 should be 205/205
V1 should be 200/200

Power dropping resistors are:
B+2 > 2k2 > B+4 > 22k > B+5 > 2k2 > B+6

Jumpering across the 2k2 between B+2/4 gives 280/285 close enough to what 102 has (and highest I can get). I understand this combines the two power stages and puts the filtering caps in parallel. Is this going to cause any adverse affects? Any reason not to replace this 2k2 with a wire?

With this jumper in place it outs V2 at 187/189 and V1 at 184/184. Adjusting the 22k will bring these voltages to where they should be. I have 18k and 20k on hand, is it better for these voltages to be a bit higher or lower than specified on these tubes?

[martin manning]

Jumpering across the 2k2 between B+2/4 gives 280/285 close enough to what 102 has (and highest I can get). I understand this combines the two power stages and puts the filtering caps in parallel. Is this going to cause any adverse affects? Any reason not to replace this 2k2 with a wire?

If you do that you lose the decoupling of the power nodes. I'd keep it to a minimum of 1k.

With this jumper in place it outs V2 at 187/189 and V1 at 184/184. Adjusting the 22k will bring these voltages to where they should be. I have 18k and 20k on hand, is it better for these voltages to be a bit higher or lower than specified on these tubes?

Adjusting the 22k is the right move. You might want to try paralleling some larger resistor across it to see how you like it vs. the lower voltage.

[dbharris]

Hey Glenn, I have that same PT in my #102 and my B+ hangs around 420-425 volts depending on line voltage. But the rest of voltages were about right with the standard dropping string. Maybe moving preamp tubes around will help.

-Dan

[jazzbass]

Hi boys,

my latest clone is a mix between #102 with EL34 power tubes. It was an inevitable choice due to the too "robust" power transformer which instead of 345-0-345 provides around 370-0-370 and due to the 3k2 50watt output transformer.
In truth the amplifier was supposed to be a #183 but the results were disappointing in terms of sound due to the voltages being too high.
I tried various methods of reducing the PT voltage but the MOS FET system caused terrible background hum. Adding a rectifier tube was impossible because the PT did not have a 5V output and the attempt to lower the voltage with diodes failed miserably.
Now I'm trying to work on the power supply string but I don't know the principles that govern it and I have to make attempts.
I obtained results even beyond expectations by inserting a 10k 5watt resistor before the choke (even too much) and now I'm trying to reduce it to 4k6.
Do you have any other suggestions? they would be very welcome.

Thanks, a hug.

Franco

[martin manning]

You must have around 500V on the plates, no? I would want the OT primary to be 5k for two power tubes, and 350V on the screens, then something close to 400V on the PI node to get back on the original Dumble PI-preamp voltages. There might be some way to make it work, but I would be inclined to get the correct PT-OT set for it.

[jazzbass]

Hi Martin,

thanks for the help, it's clear that the most correct solution is to buy a new PT......I would still like to take advantage of the PT I installed, not for sentimental reasons but just for a economic fact.
The voltage at the plates is 514 volts, the OT primary measured with a tester between pins 3 of the two EL34s is 72.8 ohms while the OT secondary is 3200 ohms, in the screens I measured 495 volts, while the voltage on the PI it is 318 volts.

I don't think I made any mistakes in my measurements.

Do you think the current PT can be saved? I was thinking of adding a resistor after the choke to dissipate some voltage, what do you think?

Thanks, a hug

Franco

[jazzbass]

Oopsss... I forgot to write that I tried to change the value of the string resistors which are now: 5k6>22K>4k7 but I can bring them back to the initial values ​​i.e.: 2k2>22k>2K2.

Franco

[dbharris]

Hi Franco,

Did you try zeners to drop the voltage? If so did you install a new cap before the diodes?

-Dan