Sentura II Schematic Errors

[patlaw]

My Sunn Sentura II that I've had since it was new has been in storage for many years. The power transformer failed. My brother replaced it, the electrolytics, and the 6.8K 2W resistors ten years ago, but the amp is still not working. A couple of weeks ago, I dug it out of storage and decided to dig through it. The circuit is simple enough, but it sure is hard finding where things are located with no parts layout diagram.

It may have been discussed elsewhere, but there are some errors on the Sentura II schematic. In particular, there is a connection missing from Pin 6 to Pin 2 of the 6AN8. Further, The plates of the 12AX7 and one side of the 12AU7 are shown as connected to "D" in the power supply. That's clearly an error as the plates should be connected to the point marked "260V". I've corrected the schematic by labeling the point as "E" and attached it. If anyone knows of other errors, please post them. If there is anything wrong with my corrections, I'm happy to fix them.

My journey to bring this amp back to life is in its early stages, but I'm sure I'll be back for help. One issue that bugs me so far is that voltage out of the 5AR4 is 604v. Since the amp is not working, that supply is probably not fully loaded, but I'm surprised it's 100 volts over the number on the schematic. Maybe that's normal or acceptable.

[52 Bill]

...The power transformer failed. My brother replaced it,...
...One issue that bugs me so far is that voltage out of the 5AR4 is 604v.

What transformer was used as a replacement?

[patlaw]

Since I don't have it here with me, I'll have to take a look and get back with you with the exact model number. If it turns out to be the wrong one, where can I get the right one?

[loudthud]

Triode Electronics makes an exact replacement. http://triodeelectronics.com/mq782.html

The filter capacitor becomes an issue in these amps because of it's 525V rating. Antique Electronic Supply sells a replacement but reliability has been questioned, some people report problems. The most reliable option is a small PCB that has radial leaded caps with equalizing resistors. These are sold as retrofits for Dynaco amps. No good place to put it in an amp with tremolo though.

[gui_tarzan]

I don't know how to thank you enough, I am rebuilding my Sentura II right now and I would have been very frustrated if it either broke something or just didn't work without these changes. I also changed the connection to the last 20/450 to "D" and the 16.5v connection on the trem/reverb board to "E", then relabeled the connection to the 2N2923 portion in the reverb section to "E" from "D" and the connection to the photoresistor to "E". I can just imagine what would happen if I were to put 260v to that photoresistor.

My schematic shows a .12pf capacitor between pin 6 and the 270K resistor that goes to pin 2 on the 6AN8 tube, and a 620K resistor off pin 8 to a 750pf cap that goes on the Reverb pot wiper terminal, and from pin 7 to a 1.5M, the 12pf cap and the 270K on pin 2 to "C", are those correct?

Also, mine as a GZ34 rectifier and it shows 500v at the standby switch. Hopefully that's what I'll get there when I put mine back together.

You don't happen to have a picture of the circuit trace side of the trem/reverb board do you? Mine's missing and I have to re-create it.

It may have been discussed elsewhere, but there are some errors on the Sentura II schematic. In particular, there is a connection missing from Pin 6 to Pin 2 of the 6AN8. Further, The plates of the 12AX7 and one side of the 12AU7 are shown as connected to "D" in the power supply. That's clearly an error as the plates should be connected to the point marked "260V". I've corrected the schematic by labeling the point as "E" and attached it. If anyone knows of other errors, please post them. If there is anything wrong with my corrections, I'm happy to fix them.

My journey to bring this amp back to life is in its early stages, but I'm sure I'll be back for help. One issue that bugs me so far is that voltage out of the 5AR4 is 604v. Since the amp is not working, that supply is probably not fully loaded, but I'm surprised it's 100 volts over the number on the schematic. Maybe that's normal or acceptable.

[patlaw]

I don't know how to thank you enough, I am rebuilding my Sentura II right now and I would have been very frustrated if it either broke something or just didn't work without these changes. I also changed the connection to the last 20/450 to "D" and the 16.5v connection on the trem/reverb board to "E", then relabeled the connection to the 2N2923 portion in the reverb section to "E" from "D" and the connection to the photoresistor to "E". I can just imagine what would happen if I were to put 260v to that photoresistor.

My schematic shows a .12pf capacitor between pin 6 and the 270K resistor that goes to pin 2 on the 6AN8 tube, and a 620K resistor off pin 8 to a 750pf cap that goes on the Reverb pot wiper terminal, and from pin 7 to a 1.5M, the 12pf cap and the 270K on pin 2 to "C", are those correct?

Also, mine as a GZ34 rectifier and it shows 500v at the standby switch. Hopefully that's what I'll get there when I put mine back together.

You don't happen to have a picture of the circuit trace side of the trem/reverb board do you? Mine's missing and I have to re-create it.

gui_tarzan, I sent the amp to a buddy to repair. Unfortunately, he was not up to the task, so I'm probably going to do it myself. The capacitor board from Triode looks to be the ticket, if it will fit. The notes for Sunn amps on the Triode data sheet are not totally clear to me. The Sunn circuit is the same as a Dynaco, from what I have been able to understand, but which Dynaco? Mark 2>?

When I get the amp back, my first objective is to identify the power transformer. It should be the right one. If not, I'm going to get the right one. My brother was quite good with tube electronics, so he obtained whatever was available and recommended, I'm sure. The fact that others complain about 600v B+ suggest that it's the right one.

Your schematic descriptions match the schematic that I have (attached in my first post) except that the cap is 12 PF, not .12 PF.

When I take it apart, I'll try to take a picture of the reverb board, if you still need it. It could be a while. A friend in Ohio has it. I'm in North Carolina.

For anyone who has purchased the Triode capacitor replacement board, please tell me what additional resistors or modifications you have had to make to use it in the Sunn Sentura II. Most importantly, the first cap after the rectifier is a 30uF 525V cap. That voltage is too low. This cap is not included with the Triode kit. Where do I get that cap? Stack a couple of 100uF 350v caps with 330K bypass resistors across each one?

[gui_tarzan]

Thanks, I did get my hands on a board thanks to a generous member here. And yes, you're right, it was 12pf not .12pf. Good catch!

When you get it back if you need help just holler. I'll get mine back out and compare notes if you want.

[g1]

Where do I get that cap? Stack a couple of 100uF 350v caps with 330K bypass resistors across each one?

Yes that is a good solution. The increase from 30 to 50uf still keeps you within the max. uf value for the first node after 5AR4.

[patlaw]

Thanks, guys. I appreciate the feedback.

[gui_tarzan]

By "stack" you mean put them in series, right?

[patlaw]

By "stack" you mean put them in series, right?

Yes

[patlaw]

I also changed the connection to the last 20/450 to "D" and the 16.5v connection on the trem/reverb board to "E", then relabeled the connection to the 2N2923 portion in the reverb section to "E" from "D" and the connection to the photoresistor to "E".

Just to be clear for anyone else reading this thread, if I read your post correctly, you and I have "D" and "E" reversed in the power supply, although it doesn't matter so long as the connecting points are corrected in the circuit.

[patlaw]

Here's an update. It turns out that the schematic I referenced above has even more errors. I have not been able to do a point-by-point comparison, but I have found a schematic that is more accurate. It's available from this link:

Sunn Schematics

In particular:

Sunn Sentura II Schematic

When I relabeled the "D" and "E" terminals, I had a 50-50 shot of getting it right. Unfortunately, the choice I made is opposite of the corrections in the diagram above. There are some differences with 12pF caps around the 6AN8. If anyone knows whether this second schematic is correct, please post.

Finally, is there any reason not to use the 40uF 600V cap from Ted Weber for the first filter after the rectifier tube? The price is right.

[patlaw]

For what it's worth, I've made additional changes to the original Sunn Sentura II schematic. My amp (1969) does not have the zener diode in the low voltage power supply section, now correctly labeled "E" to match the later Sentura II schematic. This file is hosted on a service that I may not use years from now when someone is reading this post, so I'm open to suggestions as to where it should be posted. Is there a way to host the file on this server?

Bonus question: Has anyone measured the on and off resistances of the SM-1 module in the tremelo circuit? A voltage-versus-resistance graph would be useful towards creation of a replacement, which can be done.

[g1]

This is the revised schematic