Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sean_thornton on March 12, 2016, 07:53:02 pm

Title: Need some feedback on my 5E3 design
Post by: sean_thornton on March 12, 2016, 07:53:02 pm: Hi guys,
I have a build planned. Its a 5e3 using 6K6GT valves in the output section. Node A on the power supply is 330v and Node B is 280v, should be okay there. Using a full-wave-bridge for rectification.
Does anybody see any glaring errors in my preliminary schematic?
Title: Re: Need some feedback on my 5E3 design
Post by: Fresh_Start on March 12, 2016, 09:02:04 pm: I was planning on a pre-PI master volume like that in my 5E3 build (still collecting parts). Guys here and on the music-electronics-forum.com said it would not work as expected. The "bottom" of the MV voltage divider is not ground. Instead, it's "bootstrapped" on the cathode resistor and cathodyne. No, I don't completely understand what "bootstrapped" means.

Here's the thread I started on the subject:
http://el34world.com/Forum/index.php?topic=19782.msg206407#msg206407 (http://el34world.com/Forum/index.php?topic=19782.msg206407#msg206407)

However, someone else posted their actual experience matching the predicted performance. Sorry but can't find that post at the moment.

Please don't think I'm discouraging you from trying this master volume. Just letting you know what feedback I got on the subject.

The lowered resistor values on the power rail will reduce an already meager filtering setup. Dropping from 5K to 820 will increase ripple on the power tube grid screen node. Will it produce too much noise? I don't know. Also, lower resistor for preamp node will raise plate voltage on the 5751. Not wrong, just different.

Respectfully,
Chip
Title: Re: Need some feedback on my 5E3 design
Post by: 2deaf on March 12, 2016, 11:02:39 pm: I don't know what it is, but I've been seeing that MV a lot lately. It will work, but not the the way you probably think it will. Also, it will not turn completely off.

The top power tube will be real quiet the way the circuit is drawn.
Title: Re: Need some feedback on my 5E3 design
Post by: Fresh_Start on March 12, 2016, 11:24:29 pm: Quote from: 2deaf on March 12, 2016, 11:02:39 pm
I don't know what it is, but I've been seeing that MV a lot lately. It will work, but not the the way you probably think it will. Also, it will not turn completely off.

The top power tube will be real quiet the way the circuit is drawn.

It's on Rob Robinette's website as one of many mods to the 5E3 circuit.

Chip
Title: Re: Need some feedback on my 5E3 design
Post by: sean_thornton on March 13, 2016, 01:02:16 am: I was wondering about that MV as well, I think when I build it I will run the one side to ground instead and see how that works first.
The power rail is still under scrutiny, as I said, it's a preliminary.
Title: Re: Need some feedback on my 5E3 design
Post by: sluckey on March 13, 2016, 05:50:17 am: Quote
I was wondering about that MV as well, I think when I build it I will run the one side to ground instead and see how that works first.
Simply grounding the bottom of the MV pot will mess up the bootstrapped bias on the phase inverter. The grid is not running at zero volts. You will need to add a cap between the wiper and pin 7. Also add a 1M resistor between pin 7 and the top of the 56K on the cathode.
Title: Re: Need some feedback on my 5E3 design
Post by: ac427v on March 13, 2016, 08:18:57 am: I just finished a Princeton Reverb using the same phase inverter. I could not get the Robinette MV to work so I used the traditional prePI. I added a .1 cap before the pot and used the .02 coupling cap after the pot. I am very pleased with the volume and distortion control it allows.
Craig
Title: Re: Need some feedback on my 5E3 design
Post by: sean_thornton on March 13, 2016, 09:25:42 am: Quote from: ac427v on March 13, 2016, 08:18:57 am
I just finished a Princeton Reverb using the same phase inverter. I could not get the Robinette MV to work so I used the traditional prePI. I added a .1 cap before the pot and used the .02 coupling cap after the pot. I am very pleased with the volume and distortion control it allows.
Craig

Now that design makes a little more sense. I am going to blatently steal it :icon_biggrin:

Quote from: sluckey on March 13, 2016, 05:50:17 am
Quote
I was wondering about that MV as well, I think when I build it I will run the one side to ground instead and see how that works first.
Simply grounding the bottom of the MV pot will mess up the bootstrapped bias on the phase inverter. The grid is not running at zero volts. You will need to add a cap between the wiper and pin 7. Also add a 1M resistor between pin 7 and the top of the 56K on the cathode.

Thanks for your input Sluckey, always interested in your opinions.

This is going to be a fun build but I'm not going to warm up the soldering Iron until the design is perfect. I managed to score some NOS Sylvania 6K6GT's and for some reason I felt the need for a lower power tweed deluxe.
Title: Re: Need some feedback on my 5E3 design
Post by: HotBluePlates on March 13, 2016, 03:57:05 pm: FWIW, I built a standard 5E3 copy using the Hoffman board & Hoffman-supplied transformers. No changes form the stock circuit.

I plug coke-bottle 6K6's into it all the time with no changes. I just happened to acquire a bunch of 6K6's for a standalone reverb unit I used to have, and the coke-bottle 6K6G's were too tall to fit in it (only straight-sided 6K6GT's fit).

So it's not like you need changes to use the 6K6's. I think you're making a good choice with the 0.022µF coupling caps, especially if you play hubucker-equipped guitars. The stock 0.1µF caps are a bit too muddy.