6V6 adjustments

[halfbackstrat]

As we all know i'm sure, original ODS amps are usually built on a 4x6L6 platform, often with the option to cut out two of these and half the power.
However, I know i have looked at this and have thought that even 50W is too much for my humble needs and that I would quite like to run 6V6's for a 50/25 at HP. How would I alter the power section so that you could plug 4x 6V6's in instead?
For example, #124 has 5.1k resistors running into the tubes and then 470R resistors.
How would these resistor values alter and why? Also, would any other components need to change?
Also do these resistors have a colloquial name?

Many thanks for any insight anyone could share.

Matt

[M Fowler]

You can look at the various 6V6 layouts made by Normster.

PT would be 50w rated at about 200mA-250mA with 50w rated OT at 4k.
Still looking for a 4-6V6 version but you keep the 470R screen resistors and use 1k5 grid resistors. https://tubeamparchive.com/viewtopic.ph ... 77bc6b4a2f


Mark

[ToneMerc]

As we all know i'm sure, original ODS amps are usually built on a 4x6L6 platform, often with the option to cut out two of these and half the power.
However, I know i have looked at this and have thought that even 50W is too much for my humble needs and that I would quite like to run 6V6's for a 50/25 at HP. How would I alter the power section so that you could plug 4x 6V6's in instead?
For example, #124 has 5.1k resistors running into the tubes and then 470R resistors.
How would these resistor values alter and why? Also, would any other components need to change?
Also do these resistors have a colloquial name?

Many thanks for any insight anyone could share.

Matt

Mark, I've done a couple of quad 6V6's builds. First, in a global picture will will need to modify the bias supply and tweak the NFB feedback resistor. As far as the power(DC), are you planning on using the same higher B+ voltages found the normal 100W versions or do you have plans for a scratch build, setting up something with lower a B+ voltage?

I built one as maybe a straight 6V6 amp and the other two as possible use EL34's as well. On those 6V6/EL34 amps I kind compromised by further increasing grid stoppers over what you might find in some Fender setups. As far the screen grid value I haven't experienced much tangible diferrence in the 6V6 as long as the screen voltage is few volts below the plate, so I run a smaller value.

IMHO, as far as the grid stopper value, I don't know if there is any carved in stone correct value other than something within this range; use the smallest value that doesn't cause problems and the largest value that doesn't cause problems for your circuit

https://tubeamparchive.com/viewtopic.php?t=15458
500R screen grids, 3.3K grid stoppers

https://tubeamparchive.com/viewtopic.php?t=16468
800R screen grids, 3.9K grid stoppers

https://tubeamparchive.com/viewtopic.php?t=18577
750R screen grids, 4.7K grid stoppers

YMMV

TM

[halfbackstrat]

Thanks for the feedback guys.
To be honest i don't have a build planned, while currently at University i don't think it's really an option. So i guess the question is purely academical. However. What I'd really like to do in the coming months is to design an amplifier. Perhaps a sort of smaller dumble not quite lunchbox though, in the size of the small Two Rocks, the Studio Pro and Gainmaster i think?
Anyway, the power section is going to need some serious modifications and it's probably the part of amplifier construction.

So the NFB Resistor is 4.7k for the 100W,
Is that smallest for screen grids and highest for grid stoppers? Or the other way round?

[ToneMerc]

Thanks for the feedback guys.
To be honest i don't have a build planned, while currently at University i don't think it's really an option. So i guess the question is purely academical. However. What I'd really like to do in the coming months is to design an amplifier. Perhaps a sort of smaller dumble not quite lunchbox though, in the size of the small Two Rocks, the Studio Pro and Gainmaster i think?
Anyway, the power section is going to need some serious modifications and it's probably the part of amplifier construction.

So the NFB Resistor is 4.7k for the 100W,
Is that smallest for screen grids and highest for grid stoppers? Or the other way round?

Yes, those smaller values are on the screens and the higher values are on the grid stoppers.

TM

[halfbackstrat]

Great, so what adjustments need to be made to the bias and NFB resistor? I assume drop the NFB resistor, to about half the resistance perhaps?

[ToneMerc]

Great, so what adjustments need to be made to the bias and NFB resistor? I assume drop the NFB resistor, to about half the resistance perhaps?

Here's some reading

https://tubeamparchive.com/viewtopic.php?t=17893

https://tubeamparchive.com/viewtopic.php?t=606


TM

[wjdunham]

I use a Twin supply in amps, 25W, 50W or 100W. If a customer wants either 2x6V6 or 4x6V6 I'll put a 50V high-wattage zener diode (available from Mouser) in the center tap of the secondary to drop the plate voltage down under 400V. Put a bypass switch and short the Zener if you want to go back and forth between 6L6 and 6V6 in the same amp. I leave the grids at 5.1K and screens at 470R, may tweak the GNFB if I feel the amp needs it or based on the players preferences.

Bill
www.sebagosound.com

[halfbackstrat]

That's great info Bill, thank you.
What change in sound is heard by tweaking the GNFB?

[halfbackstrat]

TM: Read those two threads over my coffee this morning, great stuff! I feel like i have a much better understanding of the NFB resistors.

Been working on an amp design today, the pre amp is based on the TR JM Sig schematic provided by Aaron (Thanks ever so much man)
and the overdrive section and power supply are from #124 for which i have to thank so much of the community.

I quite like the idea of building an amp in a smaller case, sort of like the TR SP to this end all of the switched will be done via push pull SPST/DPDT pots.
I've removed the FET, and put a 220k resistor of the B+5 line to account for this (read about that on this forum as well).

I think i'll use a 291DEX PT and a 1760K OT from the Super Reverb. (Eyed these up on Bluebell audio.

I've posted a picture of the amp schematic below, any constructive criticism is welcome and encouraged! I'd appreciate if anyone sees any mistakes or reasons why certain parts wouldn't work, they could comment

[John_P_WI]

Matt,

The bias electrolytic cap is backwards. You need the + to ground. The hum balancing resistors on the heater string are not shown grounded (Nit picking) you have the top heater string HTR1 drawn off of the xformer center tap...

Why is B+4 two parallel 10u caps instead of a single 20u as in the rest of the string? Keep things simple.

Something does not look right with the FX return ground to the bottom of the PI IMHO, also no presence?

Just the first 10 second glance at 20,000 ft....

[halfbackstrat]

Thanks for that stuff John!
I'm correcting these now, I think the backward cap, missing ground and the centre tap are just errors when using ExpressSCH (this is the first schematic i have made in it).

The two 10uF caps in parallel are as shown in the #124 schematics. I did wonder about this.
No presence control was a choice, I'm also missing a ground tag there as well!
Great input man.

[ToneMerc]

Matt I can't open that link. Are you still planning on four 6V6's?

TM

[John_P_WI]

Matt I can't open that link. Are you still planning on four 6V6's?

TM

TM, you have to download the pic from dropbox.

Only two 6v6 were shown. As we know, 4 x 6v6's are killer... More crisp, clear and don't smear like 2 x 6v6's do.... Matt, I would recommend you consider 4 x 6v6, but then your smaller chassis goes out the window..

[halfbackstrat]

TM: That's correct, i'm intending to use a 2 tube format, and go with 2 6V6 tubes, I may see if i can tweak the amp so that it can accept both, but that'll be after building.
Updated the schematic, i'll try posting that again
[img:4722:3267]http://img254.imageshack.us/img254/6602 ... teamp2.png[/img]