EZ260A Grommes 260A Project - diyAudio
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Old 9th August 2011, 07:31 PM   #1
SandyG is offline SandyG  United States
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Default EZ260A Grommes 260A Project

Just finished working on some Dynaco Mk3's and a Tubes4HiFi/Bob Latino ST120 Amp and decided to roll my own copy of a Grommes 260A.

Why the 260A? Seems an interesting amp, and that was good enough. The difference is that I'm going to make this a PCB version vs. Point to Point wiring for the heck of it, and to get some use out of all the software I have sitting collecting dust.

Most all of the schematic is the stock Grommes 260A with a few extra places for parts and circuit changes just in case someone might want to tinker with it a bit if I make some extra boards.

Attached are the 2 schematic sheets and the orig Grommes 260A. That's my jumping in point.

Go easy on me, new to diyAudio

And great forum!

Sandy
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File Type: gif EZ260A_PS_V0_1.gif (61.4 KB, 398 views)
File Type: gif EZ260A_AMP_V0_1.gif (51.0 KB, 381 views)
File Type: gif Grommes260A.gif (40.9 KB, 387 views)
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Old 9th August 2011, 08:37 PM   #2
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Great Project Sandy! I am confused however -- are you using a UL output stage as shown? Or, straight pentode mode? Your amplifier page shows UL operation, while your power supply page still shows the screen regulator in place. The original Grommes 260 was a UL amplifier, where as the A version brought the screen regulator and straight pentode operation into play.

Good luck with your amp!

Dave
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Old 9th August 2011, 08:55 PM   #3
SandyG is offline SandyG  United States
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Dave -

I left the output circuit as configurable, so if you don't want to run the 6L6GC and Pentode circuit you can still easily use it in UL mode by cutting a couple of traces and adding a couple of jumpers and of couse the right transformer. (Might need to adjust some dropping resistors values)

Some of the ideas for flexibility -
  • Solid State or Tube rectifiers
  • High voltage B+ Capable Cap
  • Ultra Linear or Pentode
  • Regulated Bias or Not
  • Regulated (well sorta) 12AU7A/12BH7A supply or not
I don't think the circuit changes much other then that that. Most of the PP common output transformers that are used have the UL taps so figured if it available why not make it a bit more flexible to allow it for those that might want to try.

That all being said, I would expect other component changes, but that is up to the experimenter to figure out as I'm not quite up to speed yet with the math behind all of this

My plan was to build it 'stock' then tinker, but if you have any suggestions I am all ears as they say!

I have the stock power transformer specs being made by the nice folks at Edcor to spec with a bit more B+ Current and an off the shelf 4.2K output tranformer and choke.

All of this is to try to fit a monblock into a Hammond 10x12x2 box

I'm way to wound up on Coffee today!

Sandy
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Old 9th August 2011, 11:56 PM   #4
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Hi Sandy -- two quick comments to consider:

1. Delete the antiquated damping control, unless your on a hunt for the specialized control and just want the feature for the nostalgia of it all! (Nah, just dump it!)

2. The HF compensation for stability will surely need adjustment when the time comes. The original OPT was very high quality units with extended HF response. Today's units typically don't have that capability so some adjustment will be needed.

Better inter-mix the decaf!

Dave
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Old 10th August 2011, 12:24 AM   #5
kevinkr is offline kevinkr  United States
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I'd make the provision for a zener string based reference for the optional screen regulator..
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Old 10th August 2011, 04:41 AM   #6
SandyG is offline SandyG  United States
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Dave -

All of that has already been removed. Too hard to find the parts for that, and don't need any more knobs

Last edited by SandyG; 10th August 2011 at 05:09 AM.
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Old 10th August 2011, 04:49 AM   #7
SandyG is offline SandyG  United States
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Quote:
Originally Posted by kevinkr View Post
I'd make the provision for a zener string based reference for the optional screen regulator..
Would that be something like a resistor from the HV1 B+ to a resistor (existing 100k in the 6L6 circuit) the screen then a string of Zeners that add up to 320V to Gnd?

Forgoing the correct resistor value, would this be the configuration of the circuit? I'm not at the schematic program or I would draw it up as to how I would 'Guess' it works

Hopefully that's all since I'm guessing I'll be tight on board space.

Sandy
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Old 10th August 2011, 09:22 AM   #8
alex mm is offline alex mm  Europe
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Default My Grommes A260 ......

Hi all,
All I can say, is that worth to build, sounds great, and a link Gegentakt-Verstärker in AB-Betrieb für EL34 / 6L6GC ....... I built ,this amp tubes last year with 6P3C Russian output tubes,
and listen to sound almost daily and becoming better.I decided to build it again this superb amplifier with GU50 as finals output tubes. Here are some pictures
Regards Alex.
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File Type: jpg FRONT+TOP+SIDE.jpg (238.6 KB, 342 views)
File Type: jpg IN+REAR.jpg (224.0 KB, 311 views)
File Type: jpg SIDE+TOP+REAR.jpg (230.0 KB, 104 views)
File Type: jpg TOP+FRONT.jpg (203.9 KB, 100 views)
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Old 10th August 2011, 10:21 AM   #9
alex mm is offline alex mm  Europe
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Default Testing amplifier .....

the image and sound here .....‪Testing Grommes 260A amplifier‬‏ - YouTube
Alex.
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Old 10th August 2011, 12:13 PM   #10
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Sandy -- simply replace the existing 220K 2 watt resistor at the control grid of the 6L6 with an appropriate zener string, and bypass it with at least a .01 uF cap. If you go bigger in capacitance, the circuit will actually provide filtering for the voltage at the output of the regulator, which is always a good thing. A 20 uF cap would be a good value, in addition to the smaller bypass cap. Remember too that you're dealing with a vacuum tube pass element here, so the zener string will likely need to be made up to equal about 310 volts, so that after the tube biases itself, the output at the cathode will be about 320 vdc.

Dave
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