Benson Monarch

[pullshocks]

What volume setting on the amp during your test? I have not turned up past 2
The Ampbooks paraphase calculator indicates a pot setting 4k ohms would be balanced. How does that line up with what you saw with the signal generator?

[T Wilcox]

Here's a video of my observations while adjusting the phase inverter pot for discussion

https://youtu.be/HLP_YHEs1II

Thanks

It's hard to tell looking at your picture it's a little blurry as I zoom in. Are you sure everything is wired up correctly there? It looks like there are 3 wires attached to the pot, there should only be two (the input and the ground). I don't have a scope, so I've just adjusted mine by ear. If I start all the way CCW, it's very noisy. As I rotate CW, I hit a point where the noise drops away suddenly. As I continue rotating CW I get a soft hum that gets louder and louder till I hit max CW rotation. It sounds best at the quietest spot.

Mike

That is pretty much in line with what I was observed except when the buzz was most quiet the tone got splatty almost like blocking distortion. There are only 2 wires connected to the pot.
I'm not sure this is problem at all other to clarify, I can get a quiet noise floor with a killer tone but I do think when the pot is set for the quietest noise floor the tone seems to have suffered some fullness and harmonics.
I will do some more testing with it and see what I can come up with

[T Wilcox]

What volume setting on the amp during your test? I have not turned up past 2
The Ampbooks paraphase calculator indicates a pot setting 4k ohms would be balanced. How does that line up with what you saw with the signal generator?

I think it was between 2-4 during that test
I will set it to 4K and see what I get . That would be at 40% rotation and I was getting a good balance between low noise floor and good tone there

[CraigGa]

So now I'm seriously considering building one of these so I have some first build questions (it's only my first valve amp build, I've been building stuff my whole life)

Looking at the diagrams kindly posted here, my PT needs to be 280-0-280v with 5v and 6.3v heater secondaries and 240v primary and the OT should be 8K primary?
I noticed that the transformer in T Wilcox's pdf is a 6.6k primary.

Is it worth paying for carbon comp resistors or should I go with modern metal oxides?

From mentions in this thread are we looking at 800 Ohm for the reverb tank input?

What gauge hook up wire do you guys use, I doubt that my regular stuff is up to the job.

I'm in the UK if anyone has any transformer recommendations.

Thanks in advance
Craig

[CraigGa]

I've just realised that transformers tend to be specced off load and I got the 280v off the schematic so there's my first mistake.

[T Wilcox]

So now I'm seriously considering building one of these so I have some first build questions (it's only my first valve amp build, I've been building stuff my whole life)

Looking at the diagrams kindly posted here, my PT needs to be 280-0-280v with 5v and 6.3v heater secondaries and 240v primary and the OT should be 8K primary?
I noticed that the transformer in T Wilcox's pdf is a 6.6k primary.

Is it worth paying for carbon comp resistors or should I go with modern metal oxides?

From mentions in this thread are we looking at 800 Ohm for the reverb tank input?

What gauge hook up wire do you guys use, I doubt that my regular stuff is up to the job.

I'm in the UK if anyone has any transformer recommendations.

Thanks in advance
Craig

My PT is 290-0-290 as well, the OT was good for either 6v6 or EL-34 for the trainwreck so I never even considered the primary Z
My voltages are approx 10% higher than the posted schem as would be expected
I started out looking at carbon comp but had trouble locating the correct sizes without using multiple suppliers so chose not to
I used a 8EB2C1B for the tank on member BobL's suggestion. It sounds great
I just used 24awg in the preamp and DC heaters, pre twisted 18awg for AC heaters
My layout was designed to utilize the existing holes from my trainwreck build so it is mirror image to the original

Todd

[BobL]

My transformer is 300 - 0 - 300 (ClassicTone 40-18028), and I used a 5Y3 to get the voltage down into range, and I'm hitting pretty dang close to spec.

[T Wilcox]

What volume setting on the amp during your test? I have not turned up past 2
The Ampbooks paraphase calculator indicates a pot setting 4k ohms would be balanced. How does that line up with what you saw with the signal generator?

After spending more time going back in forth with the PI pot this morning I settled at 4k2 on the pot. I can get the noise floor a little lower at around 3k5 but it loses something good in the tone.

[dorrisant]

Todd, usually the primary impedance that 6V6s are looking for is quite different than what EL34s (and 6L6s) require. In other words, when you use the same OT that is running two EL34s to run 6V6s, you will have to run four of them. You may be creating a severe output mismatch. All of this can be avoided by reassignment of your output taps. You may have already thought of that but it wasn't clear from your last statement.

[CraigGa]

Thanks for the advice guys.
I'll look into finding some suitable transformers.

Craig

[mikeywoll]

What volume setting on the amp during your test? I have not turned up past 2
The Ampbooks paraphase calculator indicates a pot setting 4k ohms would be balanced. How does that line up with what you saw with the signal generator?

After spending more time going back in forth with the PI pot this morning I settled at 4k2 on the pot. I can get the noise floor a little lower at around 3k5 but it loses something good in the tone.

Uncle Doug (on youtube) did a video where he compared matched vs mismatched power tube bias and showed a little mismatch to sound better. More complex harmonics with very little added hum. Sounds like your conclusion too with slightly mismatched input.

Mike

[T Wilcox]

Todd, usually the primary impedance that 6V6s are looking for is quite different than what EL34s (and 6L6s) require. In other words, when you use the same OT that is running two EL34s to run 6V6s, you will have to run four of them. You may be creating a severe output mismatch. All of this can be avoided by reassignment of your output taps. You may have already thought of that but it wasn't clear from your last statement.

Oh yeah I am aware but in this case it is iron designed for a TW Express which takes both tubes
I'm using the 8 ohm tap with an 8 ohm cab as well.
I do like to learn things the hard way though so we'll see what trouble I can get into

[T Wilcox]

What volume setting on the amp during your test? I have not turned up past 2
The Ampbooks paraphase calculator indicates a pot setting 4k ohms would be balanced. How does that line up with what you saw with the signal generator?

After spending more time going back in forth with the PI pot this morning I settled at 4k2 on the pot. I can get the noise floor a little lower at around 3k5 but it loses something good in the tone.

Uncle Doug (on youtube) did a video where he compared matched vs mismatched power tube bias and showed a little mismatch to sound better. More complex harmonics with very little added hum. Sounds like your conclusion too with slightly mismatched input.

Mike

I fell asleep watching that video last night
This does seem to be my conclusion with the PI and I think I remember that a slight imbalance was the consensus with setting the PI balance on a Dumble style amp
So far I got this build sounding great and don't know if I'll change much more on it
Time to switch my brain from electrons to woodwork

[CraigGa]

I do like to learn things the hard way though so we'll see what trouble I can get into

It's a good attitude but that smoke is expensive

[mikeywoll]

I find the switch very subtle. Look forward to trying this.
Thanks for posting.

Yep, I've found the switch to be *incredibly* subtle.

I finally grabbed a few minutes this morning to update the layout I made with all of the changes from this thread...

Attached are the new layout and a version where I marked the areas that changed.

Will update the original location of the layout as well - let me know if I'm missing something still...

I plan to try and implement these changes and see where I end up.

Hey BobL, another thing to consider...

I built the amp based off your schematic (thank you again it's awesome), but I split the power supply ground differently than your schematic. I split the power-amp ground bus from the pre-amp ground bus. The power tubes and their filter caps are grounded at the power supply ground point and the pre-amp and the 2 pre-amp filter caps are grounded through the pre-amp ground bus to a point near the input jack. I've understood this split ground scheme to be best practice and quieter (maybe others have more perspective?). Out of curiosity, I switched the ground scheme to match the schematic as posted, and it introduced a loud buzz that was preset at any volume setting.

The same thing happened when I ground the balance pot to the power-amp filter caps. It was much quieter grounded to the pre-amp.

The link below has a similar grounding scheme as I've tried to describe.

https://el34world.com/charts/grounds.htm

Mike