New Hemipool

[Noval_novice]

Hi, guys. Just finished up my first Trainwreck-inspired amp. It is the liverpool but with only 2 EL84's in the output section. I used a stock 18 watt chassis, can caps and Dana's custom made transformers from Heyboer. Also used a VVR.

When I fired her up, it was very bright (as expected) but gave a great clean-to-mean sound that I immediately fell in love with. I did have some issues with hum, but moving the second can caps ground to the preamp bus helped tremendously with that (I had it at the poweramp ground initially). Now it is very quiet except for the expected TW hiss. I have altered some values to make it suit my telecaster and try and get more of Brad Paisley tone from the monster.

My only persistent problem is an unusual phaser-like swirl that is most prominent with the gain/volume cranked. It is most noticeable when using the neck pickup and at times there is a superimposed ghost note that sounds a perfect 5th above the note played. What can I do to fix this problem? Before I started to randomly tinker, I thought I would get some educated feedback from the TAG community.

My alterations to the circuit so far:

1. Single 100pF cap for the bright switch.
2. 820R grid stopper on second gain stage.
3. 500 pF cap across R6, the plate resistor on the second gain stage.
4. Values for R7 and C8 are 68k and 0.0047 uF respectively.
5. Increased the fizz cap C12 to 500 pF.

Any help is appreciated.

[Noval_novice]

Neglected to mention tubes:

V1 = NOS 5751
V2 = TungSol RI 12AX7
V3 = JJ 12AT7
V4,5 = Mullard RI EL84's

I have made multiple preamp tube substitutions without any change. Haven't tried different power tubes, though.

[Noval_novice]

So I tried some other EL84s but the symptom is the same. Still whooshing/phasing at higher gain levels. One other problem I noted while using a chopstick is that the whole chassis is microphonic. Not onlu can you hear the tap of the stick through the speakers, but if you hit it hard, it will also elicit a ringing feedback with a frequency of about 660 Hz.

[M Fowler]

Did you mean Darin's custom transformer set?

Nice looking build.

How is the voltage to the PI?

Any change in the phasing when using different impedence ranges 4/8/16?

Mark

[Noval_novice]

Thanks for the feedback, Mark. It is less noticeable when using a 4 ohm 2x12 cab instead of my usual THD 8 ohm 2x12. Why would that be?


Here are my voltages with the VVR maxed:
B+1 332
B+2 312
B+3 253
B+4 229

I should mention that I am using all 10k resistors in the dropping string rather than the standard 9k1 resistors.

V3
Pin 3,8 69
Pin 1 165
Pin 6 174

V2
Pin 1 207
Pin 3 2.2

V1
Pin 1 149
Pin 3. 1.7
Pin 6 134
Pin 8. 1.2

[gary]

is it a "whole-amp" vvr implementation or are you hitting the output stage particularly hard? Does it happen with vvr running at full voltage?

[Cantplay]

With very high gain, 12ax7 can be microphonic. I see this often in phono stages.

If you have any 5751 try them one at a time.

I also don't see a grid stopper on input jack, or am I missing seeing it?

John

[Noval_novice]

I am using a 5751 for V1. The VVR scales the whole amp. There is no input grid stopper on the Liverpool schematic, but I am considering making the following changes to the amp:

1. Rewire the preamp for a paralleled pair of triodes for V1, moving other gain stages to V2.
2. Add 10k grid stopper on V1.
3. Balance the PI.

I am hoping the following changes will thicken the tone up a tad and reduce some of the excessive gain, but I am open to other suggestions.

[selloutrr]

Silly question but did you meter / test all the components before you started the build? the Swirl could easily be an out of spec part or as pointed out a questionable tube.

[Zippy]

I am using a 5751 for V1. The VVR scales the whole amp. There is no input grid stopper on the Liverpool schematic, but I am considering making the following changes to the amp:

I am hoping the following changes will thicken the tone up a tad and reduce some of the excessive gain, but I am open to other suggestions.

1. Rewire the preamp for a paralleled pair of triodes for V1...

That will add gain.

[Noval_novice]

I don't have a capacitance meter, but it's probably time to invest in one. Will check to make sure my PS caps are good. I always measure my resistors.

[JoeCOOl]

What value did you use for cathode resistor on power tubes? Did you double value to account for two tubes?

your problem sounds like extreme crossover ditotion, so check output bias and negative feedback network.

[Noval_novice]

So, I think this is finally resolved. I was uncertain if the phasing, ghosting was due to a parasitic oscillation from my poor lead dress or due to crossover distortion, but I was able to reduce the effect to a minimal (possibly normal) level by balancing the PI and increasing the tail resistor to 56k. Now I have just a very subtle (and not unpleasant) swirl when playing at the highest gain settings. Thanks, everyone!

[zambo]

I also had this problem. Tried removing the bypass cap on power tube cathode r and it did the trick. lost a little tone doing it but it still sounds sweet by any standard.

[zambo]

I built the same amp, how is the vvr and did you build it or by one from hall amplification? very curiouse about the vvr!