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Old 14th August 2011, 02:12 AM   #901
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Hi Printer2;

something is weird around your source follower. It does not have a gate bias, and I think B+ voltage to power it is too high.

Also, I suggest you to utilise screen grid current to add some sagging through B+ for preamp.
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Old 14th August 2011, 02:35 AM   #902
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Quote:
I hope those exposed primary taps on the output transformer are going to be shrouded.
Hi Daryl, I should have used the word 'breadboard', not 'prototype'.
That arrangement is what I used to develop my 6AK5 preamps on previously, and I just took out a tube socket and screwed on the PA tfmr. My plan for the 'Lord of the Rings' is to have all the trfmrs inside and only the tubes poking thro the top. Sockets and switches and knobs along one side.
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Old 14th August 2011, 02:41 AM   #903
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Hi Printer2,
glad you bounced - I am an ex motorcyclist and can feel your rash.

Love your switching options, I did take your pp to self split switch for my amp on yesterday's post.

This thread has been great for sharing ideas.

Keeping the volume and the complexity down (switches don't count) has been as much a challenge as the $100.

Cheers

Last edited by djgibson51; 14th August 2011 at 02:42 AM. Reason: spelling
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Old 14th August 2011, 03:09 AM   #904
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Quote:
something is weird around your source follower. It does not have a gate bias,
The source follower will work fine if you remove the .022 cap on the gate and replace it with a short. It is the same design I use, and tonight I decided to turn up the fun. Since I already know how violently the electrolytics that I am using explode on 600 volts, I decided to stay within the ratings. I set the power supply on 450 volts for my 450 volt caps. So far the only thing I blew up is a resistor. It was the stinkyest resistor I ever fried. I did the proper Tubelab thing and just put in a bigger resistor.

I am making an amp design that can be built for several prices. Some will fit this budget, and some will not. It is all a matter of what parts you stuff into the board, and what tubes you use. SO.....since there has been some talk about violating the rules a bit, I decided to wire in a real OPT, connect up a variable power supply, and roll some tubes through this thing. NO other changes were made.

The budget consious choice is 6BQ7/6BZ7/6BK7 in all 3 preamp sockets and 6EZ5's or 6W6's in the output sockets. 6BQ7's are $3 each and 6EZ5 were on the $1 list but are $4 now. The 6W6 is $3. The fun starts at 250 volts on the B+ and it rocks at 350 or maybe....just a bit more. Power is 10 to 20 watts depending on supply voltage. If a reasonable OPT and an Antek 05T230 is used for the power transformer it will fit into the under $100 budget.

A 12V6 tube is $3 and it fits the budget and sounds real good. This allows 12 volt dual triodes too. Of course you can bust the budget and use 12AX7's but there is actually too much gain. There are several other possibilities.

I rolled about 50 tubes through this thing. I know what's going into my amp, but the tubes cost more than the rest of the amp! When I was experimenting with the series string heaters I tried 26AQ8/UCC85 tubes. So for the 6 volt version I plugged in some ECC85's they ROCK but cost $15 EACH! Of course to get that Marshall cranked to 11 sound you got to stuff the amp with EL34's and set the power supply for 450 volts. I got my Sovteks for $10 each but I guess they cost more than that now.
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Old 14th August 2011, 03:48 AM   #905
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Quote:
Originally Posted by Wavebourn View Post
Hi Printer2;

something is weird around your source follower. It does not have a gate bias, and I think B+ voltage to power it is too high.

Also, I suggest you to utilise screen grid current to add some sagging through B+ for preamp.
Shoot, threw in the MOSFET and forgot to remove the capacitor.
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Old 14th August 2011, 04:10 AM   #906
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Wow tubelab! This sounds like great fun! so many possibilities! When you finalize the design are you thinking about offering boards or kits? I really like how this thread is getting everyone's juices flowing!
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Old 14th August 2011, 04:15 AM   #907
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Originally Posted by djgibson51 View Post
Hi Printer2,
glad you bounced - I am an ex motorcyclist and can feel your rash.

Love your switching options, I did take your pp to self split switch for my amp on yesterday's post.

This thread has been great for sharing ideas.

Keeping the volume and the complexity down (switches don't count) has been as much a challenge as the $100.

Cheers
Good someone has use for the split to p-p idea. I wanted to use it but in the end I saw another shiny object. I might go back to it after I see if I can get this working.

Not sure if I mentioned it before but the same circuit can be used for self split, push pull, for a SE amp using the same board. Just a part or jumper change, in the case of the SE amp a different OT but even that could be worked around by biasing the bottom p-p tube constantly on. What I wanted to do is not give a specific design but rather a platform to play with. I also wanted to see what a 70V transformer can do in p-p and in SE, I already converted one with an air gap.

Hopefully tomorrow I will get some more construction done. It was a bit too awkward today so I spent the time doing the schematic. Not sure if the MOSFET is necessary, I am not looking to make a gain monster, but I will leave space for it. Also the idea for this amp is to put a booster in front of the amp to drive the pentode harder if need be.
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Old 14th August 2011, 02:27 PM   #908
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When you finalize the design are you thinking about offering boards or kits?
A lot depends on the attitude of my wife when she finally returns home. She has been away for over a year, and on and off before that caring for her mom. Since I was there for the funeral another relative had died, and a third in in the hospital with cancer.

Sherri has been under a lot of stress due to all this BS, but she seems to be in favor of closing Tubelab Inc. down since it consumes a lot of time and doesn't make any money. With the crummy economy board sales are very slow now.

It's too soon to tell, but I would like to do boards for the most popular designs.

Quote:
Not sure if the MOSFET is necessary, I am not looking to make a gain monster, but I will leave space for it.
I included mosfet buffers for the output of every stage in the first pass PC board. The amp currently has 2 mosfets. One drives the volume control, and I am not sure it really does much. I am going to try a 1 Meg volume control without the mosfet and see if it cuts too much treble or loses gain.

The mosfet feeding the tone stack is a keeper. Simulation of the tone stack shows it is better behaved with a low source impedance. Testing with and without the mosfet confirms this. I use a 47 K 1 watt resistor for the source to ground resistor and an LND150 mosfet in the TO92 package. It is the cheapest HV mosfet in the Mouser catalog and works fine.

A gate stopper resistor is needed to keep my design from oscillating at 24 MHz but I am using VHF TV tuner tubes. I am using a 27 K resistor where your coupling cap is.
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Old 14th August 2011, 03:16 PM   #909
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Originally Posted by tubelab.com View Post
I included mosfet buffers for the output of every stage in the first pass PC board. The amp currently has 2 mosfets. One drives the volume control, and I am not sure it really does much. I am going to try a 1 Meg volume control without the mosfet and see if it cuts too much treble or loses gain.

The mosfet feeding the tone stack is a keeper. Simulation of the tone stack shows it is better behaved with a low source impedance. Testing with and without the mosfet confirms this. I use a 47 K 1 watt resistor for the source to ground resistor and an LND150 mosfet in the TO92 package. It is the cheapest HV mosfet in the Mouser catalog and works fine.

A gate stopper resistor is needed to keep my design from oscillating at 24 MHz but I am using VHF TV tuner tubes. I am using a 27 K resistor where your coupling cap is.
Gate stopper, glad you mentioned it, another thing I overlooked. I also see that I did not do a complete voltage doubler, did a bunch of cut and pastes from different designs and missed a few things. I also found the software I have actually does have a Mosfet symbol, I did not have to use the J-Fet.

I will be using a IRF820 rather than a LND150 as you can get them or a replacement all over, even EB*y while as I had a hard time finding the LND150 anywhere other than Mouser for a reasonable price. Picked up a bunch from Mouser but shipping would have been too much if I only bought a couple of parts. Originally I would have gone with the LND150 as the first stage but semi's are not allowed for gain stages.

Might go for a dual pot on the post PI volume control, you can pick them up online for about what a single costs. Not sure if it is necessary though.
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Old 15th August 2011, 12:37 AM   #910
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It looks like I'll be completing the amp after all. I started on the cab today, and had to stop to make a drilling jig for the dowels. Cab is assembled with two dowels on each end of each horizontal section, except the miter cut 3/4" frame around the speaker board.

There is an additional horizontal brace across the back bottom making a tub for the Reverb Tank.
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