pre build critique needed on ODS schematic

[Tonegeek]

12/07/06 late: changed OD drive to audio, tamed the bright caps and upped the 500pf CL2 grid cap per Dogears. Also eliminated the bypass caps on the dumblelator per some comments I read elsewhere.

12/07/06 worked on the bias supply (going with precision PS)
*note shematic shows 6L6 power, but I am going with EL-34s to start out.
UPDATE:
(added the loop and worked on the bias values. also put back original 500pf treb cap in OD) Still shows the local FB, but I am going to leave that off per a suggestion and it also seems I may not need the bright cap in the loop either.) w

MY ORIGINAL POST:
I got this schem from the files section and changed a few things. Please critique and comment. I am going with the early PS unless talked out of it. I lowered the B1 caps to 100uF per a post by Gary. In the pre-amp, I pulled back on the OD treble cap and the CL2 grid RC bypass cap for no special reason, just had a gut feeling about the sound. also added the LNFB just for grins.

Not sure about my bias PS resistor values since I am using 2 trim pots. I need versatility so I can use EL-34 or 6L6's.

Only thing missing from the schem is I am going with an onboard dumbleator, w/ true bypass. So, there should be one more section added for that tube to the PS, etc.

I welcome comments on these issues:
How is the response affected with a choke vs no choke?
What is the effect of the local feedback circuit vs no LFB?
Anyone see any problems with using the Heyboer TO-60 OT from http://www.allenamps.com/parts.html and a 50W Marshall PT ?



thanks,
whit

[Bob-I]

This is a good schematic to start with. The PS rail is difficult to determine before completion because it depends on the absolute B+ and current draw by the preamp. You've got good starting points.

I'd drop the bright sw cap to 120pF as I find 270pF to be overly brittle. Skip the LNFB for an HRM amp, it's better suited for a Hybrid. You might want to up the HRM treble cap to 500pF, I like that value in mine. I also used 220k grid loads on the power tubes connected to the bias, you're showing 200. I'm not sure what that'll change. Make sure your bias test resistors are 1% 1 ohm 2 watt.

Good luck with the build.

[heisthl]

I doubt if the bias will work for the el34s you might try a 30k instead of the 15k and use a switch to parallel in another 15k for 6l6's (just guessing at the values - use clip leads to see what works).

[ayan]

I got this schem from the files section and changed a few things. Please critique and comment. I am going with the early PS unless talked out of it. I lowered the B1 caps to 100uF per a post by Gary. In the pre-amp, I pulled back on the OD treble cap and the CL2 grid RC bypass cap for no special reason, just had a gut feeling about the sound. also added the LNFB just for grins.

Not sure about my bias PS resistor values since I am using 2 trim pots. I need versatility so I can use EL-34 or 6L6's.

Only thing missing from the schem is I am going with an onboard dumbleator, w/ true bypass. So, there should be one more section added for that tube to the PS, etc.

I welcome comments on these issues:
How is the response affected with a choke vs no choke?
What is the effect of the local feedback circuit vs no LFB?
Anyone see any problems with using the Heyboer TO-60 OT from http://www.allenamps.com/parts.html and a 50W Marshall PT ?



thanks,
whit

Hi Whit,

On the PS, the three dropping resistors on a real HRM amp were 3K, 15K, 10K. While it is true that one may want to tweak the PS to achieve the desired voltages, Dumble seems to have akways done that by selecting the proper transformer or maybe toying with the first dropping resistor. The last two were the same across the board in all amps: 22K and 2.2K in the early 80s amps, and 15K and 10K in the later amps.

Cheers,

Gil

[Tonegeek]

I made a few component adjustments and just need another set of eyes, mainly on the power supply and bias supply values. My math on the bias voltages may be off a bit because I did not factor in the load. I want the bias to be flexible enough for 6L6 or EL-34's. I am using a Marshall PT.

Is 50uF (total) going to be OK for B+1? I don't want the amp to be too stiff.
thanks,
whit

[dobbhill]

Should the "drive" control be an audio or linear taper? I thought audio, but I might be wrong, er, I mean misinformed..
D

[Bob-I]

Should the "drive" control be an audio or linear taper? I thought audio, but I might be wrong, er, I mean misinformed..
D

Apparently Dumble used linear. I tried it but personally I like audio. It's really a preference as to where you like to set the knob. I found the linear to be too sensitive at lower settings.

[Bob-I]

I want the bias to be flexible enough for 6L6 or EL-34's. I am using a Marshall PT.

Don't forget that EL-34's have roughly half the impedance of 6L6's. You'll have to double the load or change the impedance setting you use.

Is 50uF (total) going to be OK for B+1? I don't want the amp to be too stiff.
thanks,
whit

Yes, 50uF is good. I tried more, 110uF and liked 50uF better.

[Normster]

I want the bias to be flexible enough for 6L6 or EL-34's. I am using a Marshall PT.

Don't forget that EL-34's have roughly half the impedance of 6L6's. You'll have to double the load or change the impedance setting you use.

??? Unless I'm misreading the tube data sheets, a pair of 6L6s at 450V has 5,600 Ohms plate to plate and a pair of EL34's at the same voltage has 5,000 Ohms. Did you mean 6V6's?

[dogears]

Here are some things to change

Up the bypass cap on CL2 grid to 500pf

Definately use an audio taper on the OD gain pot. The linears were used on 100k amps. This is a 250k. Needs to be audio. Try both and see I am right.

No more than 12pf bright cap on the OD master.

No more than 22pf bright cap on clean master.

Try 150k on the grid of OD2. Some had it instead of 180k. To taste.

[Tonegeek]

Here are some things to change

Up the bypass cap on CL2 grid to 500pf
Definately use an audio taper on the OD gain pot. The linears were used on 100k amps. This is a 250k. Needs to be audio. Try both and see I am right.
No more than 12pf bright cap on the OD master.
No more than 22pf bright cap on clean master.

I never saw a bright cap I liked! I would not be surprised if I end up taking them all off,but we will see when I hear it. I updated the schematics with the suggestions, except holding off on the OD grid resistor. now if I can just find a chassis for 4 pre amp tubes...