Benson Monarch

[T Wilcox]

the readings on the schem are read to ground or 0V

[dorrisant]

Maybe this will help...

[CraigGa]

Yes that helped, it's only half wave rectified!

Thanks
Craig

[T Wilcox]

It's a full wave rectifier, scroll down to rectifier on the following link
https://robrobinette.com/How_Tubes_Work.htm

Based on Dorrisants link, you would multiply the full AC voltage (measured across both secondaries) of 560 x .71 rather than the 280 x 1.414 which is measured one leg of the secondary to ground

Someone correct me if I am wrong

[brewdude]

As I understand it, and feel free to correct me if I’m wrong.

I think if you measure the ac you will find 560v. However, you would not multiply that by 1.14 (w/a full wave BRIDGE) to get expected outcome, due to using the center tapped PT with full wave rectifier. More like 0.71—in an ideal world, anyway.

[CraigGa]

Thanks guys, I get it now.
So is 150mA about right for this build?

Craig

[mikeywoll]

Thanks guys, I get it now.
So is 150mA about right for this build?

Craig

That should be more than adequate. I calculated between like 80 and 110 mA depending on output transformer impedance. You can use a power transformer that supplies less current and will get more sag, while a power transformer that supplies 150 mA will feel more stiff. Your rectifier choice and fine tuning to filtering can dial in the feel one way or the other once you get the amp built. This stuff is all relative to the other components in the amp. They all work together to give you the feel and tone. We don't know exactly the values in the real amp, but it seems to play a lot like a deluxe. Using similar iron should get you in the ballpark, hence several folks using 5e3 transformers.

I gutted a blues Jr for mine. That power transformer is rated at 68 mA and output transformer is 6.9k@ 8ohms. It is pushing the limits of sag, for me. Any more would be too much... and that is with a solid state rectifier. So, it's all relative.

Mike

[CraigGa]

So if you used a transformer for a 5e3 would you use a more lossy rectifier to keep the voltages inline with the schematics?

[mikeywoll]

So if you used a transformer for a 5e3 would you use a more lossy rectifier to keep the voltages inline with the schematics?

I won't claim to be an expert on vintage amps, I've seen all sorts of numbers thrown out there for 5e3 transformers. Everything from 260-0-260 up to 380-0-380. The lower end of that range with a 5ar4 rectifier would hit pretty close to the voltages on the schematic. You could use a higher voltage transformer with a more lossy rectifier to also hit the voltages, as you were asking. That is also the scenario that TWilcox was describing earlier. Each scenario may potentially feel and sound a little different from each other when playing the amp.

Mike

[dorrisant]

This will get you wherever you need to go...

https://ampgarage.com/forum/viewtopic.php?t=27035

[alexrussomusic]

Hey all - got one of these recently and it seems to have a little more hum than I'd like, so I cracked open to look for obvious suspects...and there were some surprising circuit differences. v1b had a 1k cathode instead of 2k2, the split plate was a 5k8 instead of 10k w/ a .1uf feeding the PI. Also completely different power supply string with an 88k supplying the preamp node??? its 32 uf->1k-> 32 w/a 47uf (? its hot glued value side down) --> 88k --> 47uf and that supplies both the preamp & phase inverter.

Oddly enough even with all these changes, the voltages are pretty spot on in the preamp.the power nodes are ~10v higher & the power tube bias seems a little off (16.6 vs 19.6). EDIT - just put the original 6v6s back in and the voltages all came to spec. I think I bought a crappy pair of 6v6s. Way more headroom / less hum now. I'm thinking of changing the 5k3 split to the more standard 10 and see what difference it makes and maybe swapping out the power tubes. Maybe Chris can chime in?

FWIW the PI balance is set at ~3.26k.

Just gotta say, this thing sounds GREAT cranked. SO MUCH FUN. Left the pedals at home last week and didnt miss them. Big kudos to Chris & co for making a modern classic.

[mikeywoll]

Well, curiosity got the best of me... I tore out the eyelet board and rebuilt PTP. I ran into some of the same issues with the PTP build as I did with the eyelet layout and have some observations.

But first a couple pics...

PXL_20210503_030054248.jpg

PXL_20210420_044706289.PORTRAIT_resize_37.jpg

I didn't have a picture of the eyelet build with the correct power supply filter caps, but I ultimately built the amp to identical schematics, just the 2 different approaches.

I built the amp in the chassis and cabinet, and scalped the transformers and a few parts from a Blues Jr tweed model. This transformer lacks a center tap, but with a bridge rectifier (diodes scalped from the BJr circuit board) it puts out 335V at the power tube node. All of the voltages match the schematic well. The reverb tank is an 800 ohm tank and works (it's not a great tank but gets the job done). I added a VVR (I'm a fan), which causes predictable problems with the cathode follower driven reverb when turned down, but overall I like the VVR with this amp.

My observations...

1) Both layouts allow quiet playing with the volume at 0. Turns out my volume pot doesn't go to 0. That one drove me crazy.

2) I ended up needing exactly the same tweaks to block RF in both designs. 10k grid resistors on the reverb tube was a must with the BJr tank.

3) PTP was much easier to build and fits much better in the small chassis I used. I struggled with ground loops on the eyelet build that didn't occur with the PTP build.

4) There is some nuance to grounding that isn't obvious from the layouts. I had a ton of hum until I split the power supply grounds, grounding the pre-amp filter caps through the ground bus on the pots. Also had to make sure the phase inverter trim pot was grounded by itself. It is grounded alone in the photo of the original amp. These issues occured with both layouts.

5) There was a graininess to the overdrive and I found the treble to be a little spikier with the eyelet build. It was almost like a subtle, static like vibration riding on top of the overdrive when the amp was cranked. This seems to be gone and the treble tamed slightly with the PTP build. This could be in my head and could also have been due to technique, but I think the PTP build is better in this regard.

6) The BJr transformer is underpowered (68mA) and works really well with this amp, sagging in a pleasant way without farting out. There is great touch sensitivity and compression when you dig in. There is a ton of bass, and the bottom end breaks up nicely when driven. I don't think either layout performs better than the other here.

Overall, this amp is fantastic. This is my go to "quiet playing after the family goes to bed" amp. I dial down the VVR a little and plug into an attenuator to bleed off another little bit of volume... Amazing touch sensitive break-up at quiet voice volumes.

[T Wilcox]

Very nice Mike!
I took mine out of the cab yesterday to swap in a GE 5u4 tube which put my voltages pretty close to the schem. I was 20-30 volts high all around with the GZ34.
While testing it I did not have the reverb pan plugged in and noticed the volume knob does go quiet without the pan plugged in
Other than that observation I didn't mess with it much since I don't really need it to be whisper quiet.
I did add a 4.7uF to that extra boost switch I had installed. In boost mode it bypasses the 2K2 on the second gain stage. It will thunk when switched on but it opens things up more and sounds glorious. I found myself leaving it on. If you try it out let me know what you think

[T Wilcox]

Hey all - got one of these recently and it seems to have a little more hum than I'd like, so I cracked open to look for obvious suspects...and there were some surprising circuit differences. v1b had a 1k cathode instead of 2k2, the split plate was a 5k8 instead of 10k w/ a .1uf feeding the PI. Also completely different power supply string with an 88k supplying the preamp node??? its 32 uf->1k-> 32 w/a 47uf (? its hot glued value side down) --> 88k --> 47uf and that supplies both the preamp & phase inverter.

Oddly enough even with all these changes, the voltages are pretty spot on in the preamp.the power nodes are ~10v higher & the power tube bias seems a little off (16.6 vs 19.6). EDIT - just put the original 6v6s back in and the voltages all came to spec. I think I bought a crappy pair of 6v6s. Way more headroom / less hum now. I'm thinking of changing the 5k3 split to the more standard 10 and see what difference it makes and maybe swapping out the power tubes. Maybe Chris can chime in?

FWIW the PI balance is set at ~3.26k.

Just gotta say, this thing sounds GREAT cranked. SO MUCH FUN. Left the pedals at home last week and didnt miss them. Big kudos to Chris & co for making a modern classic.

Thats very interesting!, Is yours a reverb version? You didnt by chance snap any pics of the inside? I'm still curious about the power section. More how things are ran than what values

[mikeywoll]

Hey all - got one of these recently and it seems to have a little more hum than I'd like, so I cracked open to look for obvious suspects...and there were some surprising circuit differences. v1b had a 1k cathode instead of 2k2, the split plate was a 5k8 instead of 10k w/ a .1uf feeding the PI. Also completely different power supply string with an 88k supplying the preamp node??? its 32 uf->1k-> 32 w/a 47uf (? its hot glued value side down) --> 88k --> 47uf and that supplies both the preamp & phase inverter.

I'm also very interested in seeing the power supply . This would make sense if...

30uf -> 1K -> 20uf (47 + 32 in series) -> 8.8k -> 47uf

EDIT*** I realized after posting that the order here doesn't make sense, it's probably something like 20, 47, 30 ***

For a head (no reverb) you would get similar voltages and filtering to the 20,47,20,20 power supply we are running on the reverb builds. That should supply enough filtering to run the PI and pre-amp tube together. A few people stumbled onto a very similar formula when trying to sort things out initially. That also looks very similar to what Aaron initially traced out and Chris Benson commented on... If you recall, we were thinking 30,30,30 with 1k and 10k dropping resistors.

There's no way it's an 88k resistor in the power supply. That would drop way too much voltage to be viable.

Mike