Ezg-50 circuit changes?

[Lumpylipton]

I know the ezg-50 is described as basically an ab763 super reverb trem channel with a mv in place of the mixer resistor, but does anyone know if there are part value substitutions? From the gutshots I've seen, it seems to be exactly part for part the same as the super, but of course I can see parts values. Is the very strong lowend just from the special tranny, or are there other differences that explain it?

[M Fowler]

I haven't looked at the values but I assume it is blackface since Doc said it was but uses a specially designed PT by Ken Fischer and OT is different, states not normal specs used for a BF amp.

Kind of reminds me of an Allen Amp circuit with two knob reverb and using 6L6's.

[pdf64]

Ha, the strong low end comes primarily from a linear taper tone controls; once the standard taper pots are fitted, the resultant tone ends up much more in the ballpark of user expectation.
However, even then the tone seems more 'solid' than regular BFs; my guess is that the extra B+ node and 47uF B+ caps all through may be a contributory factor.
Another contributory factor may be the OT; it's big! Only a bit less iron than a TR OT. The primary is ~5k, which may suit the tubes now sold as 6L6GC better.
The other key difference to BF scheme is the reverb dwell is a 100k pot but it still uses a 500pF coupling cap, so a lot less low end going into the tank, which is a small type.
It's a really good reverb.
I made a few minor mods to my buddy's EZG50 (including adding a bright cap) and it's now his main gigging amp; previously he just couldn't get a tone that wasn't overly bassy.
Pete

[M Fowler]

I bought a blemish EZG50 head cab from Dr Z two years ago, Mike understandably said no to selling me a chassis so I had one made along with a faceplate but still haven't put the amp together. Too many other projects to get done. I could make a board and drill/punch the chassis to at least get it ready for assembly.

[Lumpylipton]

Thx guys, great info. Pete, what do you mean by additional b+ node? A standard bf has 4 (pre choke,post choke, post 1k resistor and post 4.7k resistor), does the ezg have 5? What stage is connected to it? And through what value of dropping resistor? Can't see the can cap from the gutshots, so i missed this entirely.
Warren

[M Fowler]

Hard to tell how many B+ nodes. The reservoir caps are in series plus two can caps under board. But I'm only counting three B+'s feeding the board.

Mark

[pdf64]

I'll tidy up the scheme that I sketched and post it.
As mentioned, I made a couple of mods, so I'll try to amend it to the pre-modded condition.
There's 2x 32uF F&T can caps (sorry I stated they were 47uF earlier) in addition to the series 100uF reservoir caps, so 5 nodes in total, one each for V1, V2, LTP / reverb driver, screens, reservoir.
Droppers (in above order, last to first) 10k, 22k, 4k7, choke (155ohms)
The OT primary was 5k5.
In stock form, the power amp doesn't take well to being cranked, unpleasant blocking distortion.
0.1uF coupling caps between LTP and power tubes, plus the bias supply impedance is rather high.
Pete

[Lumpylipton]

Looking forward to seeing that Pete. Having another node and different dropping resistors will result in different anode voltages at each stage, very interesting.

[M Fowler]

Pete,

Thanks for the info.

Mark

[Lumpylipton]

Pete
Did you also measure the b+ voltages at the different taps?

[hywelg]

I have a draft schematic of the preamp if anyone wants a look. PM only as I'm not happy about a public posting, I did it mainly for my own interest and I respect Mike Zaites work, even though this amp is very very similar to a Super.

I dont get chance to crank mine so have't had experienced the blocking distortion mentioned. I would guess it was very very loud at that point.

[pdf64]

Sorry, I'm not getting around to re-drawing my notes, so in the meantime I've scanned in the raw version.
The sketch shows a partly modded condition, with bright cap, 2nd stage cathode bypass and global NFB switches added, the 'tizz' cap removed (the 10pF across the 3M3 'dry' mix resistor) and lowered coupling caps to power tube control grids.
Also added series Si diodes to the rectifier plates, to maintain operation in the case of the tube shorting, and grid stoppers (generally 10k) on the pre-amp tube sockets, as bypassing the 2nd stage cathode and turning off the NFB could tip it into oscillation.
It also shows an idea I had of moving the standby switch to the rectifier heater.
I went on to replace the 1M lin MV with a 250k lin (old bass control), fed via 220k, and reduce the coupling cap to the LTP to 500pF.
Also moved the standby switch to the screen grids, in order to remove stress on the rectifier when operated. That was easy to do on the amp as implemented and it works very well.
The viewpoint I'm taking is that DrZ amps offer fantastic build standard and are made in the USA, I can't see how anything comparable could be retailed for a better price; the scheme is only a part of the offer and I'm showing a (significantly) amended version of it.
The amp came to me initially due to suspected fuse blowing, hence line current measurements; it turned out that it was just the pilot lamp blown.
Sorry it's upside down.
Pete

Edit; another potential contributory factor to the tone being different to, say, a Super Reverb, is that the pre-amp B+ voltages in the EZG50 are lower, about 300V, compared to >400V in a SR.

[Peeping tom]

Ha, the strong low end comes primarily from a linear taper tone controls; once the standard taper pots are fitted, the resultant tone ends up much more in the ballpark of user expectation.
However, even then the tone seems more 'solid' than regular BFs; my guess is that the extra B+ node and 47uF B+ caps all through may be a contributory factor.
Another contributory factor may be the OT; it's big! Only a bit less iron than a TR OT. The primary is ~5k, which may suit the tubes now sold as 6L6GC better.
The other key difference to BF scheme is the reverb dwell is a 100k pot but it still uses a 500pF coupling cap, so a lot less low end going into the tank, which is a small type.
It's a really good reverb.
I made a few minor mods to my buddy's EZG50 (including adding a bright cap) and it's now his main gigging amp; previously he just couldn't get a tone that wasn't overly bassy.
Pete

Hi,
I'm interested in this circuit...can you tell me how the MV work here in Dr. Z ezg 50?
Are there any other differences except for the reverb dwell pot?
Thank you very much!

[Peeping tom]

I have a draft schematic of the preamp if anyone wants a look. PM only as I'm not happy about a public posting, I did it mainly for my own interest and I respect Mike Zaites work, even though this amp is very very similar to a Super.

I dont get chance to crank mine so have't had experienced the blocking distortion mentioned. I would guess it was very very loud at that point.

Hi
I'll appreciate if I can have a look of the preamp schematic!
Thank you very much!
my email: mihailsivanov@gmail.com

Best Regards!

[pdf64]

Ha, the strong low end comes primarily from a linear taper tone controls; once the standard taper pots are fitted, the resultant tone ends up much more in the ballpark of user expectation.
However, even then the tone seems more 'solid' than regular BFs; my guess is that the extra B+ node and 47uF B+ caps all through may be a contributory factor.
Another contributory factor may be the OT; it's big! Only a bit less iron than a TR OT. The primary is ~5k, which may suit the tubes now sold as 6L6GC better.
The other key difference to BF scheme is the reverb dwell is a 100k pot but it still uses a 500pF coupling cap, so a lot less low end going into the tank, which is a small type.
It's a really good reverb.
I made a few minor mods to my buddy's EZG50 (including adding a bright cap) and it's now his main gigging amp; previously he just couldn't get a tone that wasn't overly bassy.
Pete

Hi,
I'm interested in this circuit...can you tell me how the MV work here in Dr. Z ezg 50?
Are there any other differences except for the reverb dwell pot?
Thank you very much!

See the schematic I linked/attached previously; it's a simple type4 (before phase splitter) master volume, same as a regular 2204 Marshall or SF Fender.
There is no significant degree of overdrive available from the preamp, very different to a 2204.

As for the other differences from eg an AB763 Super Reverb, I think I covered them all in my previous posts in this thread?
Let me know if you need anything explaining.