100w ods

[zimm]

Here is my Non-HRM amp, there is nothing inventive here, this amp is based off of info provided by all the great members here. Id especially like to thank Funk, Dogears, Andy for the MM power supply tips, Tony for helpful advice, Gil and Henry for help in the past as well. Without you guys this would have never happened!

I have to open it up to get the current voltages but last I checked v1a was 188V, v2a is around 200, and v3 is around 295-305 or so.

I had EH 6ca7s in there, they sounded OK but I had one short the screens and some other odd stuff. I pulled them and went with Sovtek 5881's as per Funk and Dogears suggestions. I am much happier with these.

Iron is from a musicman hd130, 1972ish that I got from CL. I got a deal cuz it didnt work and took a gamble that the trannys were OK. I checked it out and it turned out that the choke was fried. After I got the MM running I pulled the trannies and put them in this build.

Power stage layout is the one suggested by Andy, using the zener diodes to help protect the preamp tubes. Probably not necessary since I went with a traditional power stage but I wasnt/ still not sure, if I would change that in the future. Screens are 350V. Dropping string is standard but I did have to raise the first resistor from 3K3 to 4k7 i think. There are also two 450V/100uf caps under the filter board, so filtering is set up series parallel to boost filtering. Have not hooked up the DIN for the FS yet, have a regular jack there for the time being.

Chassis was made from 1/16" aluminum, cut bent and welded by me. Labels were made with a p-touch labler, black on clear tape. Cabinet is made of pine, mitered, and "tolexed" with 3M sprayable rubberized undercoater form the auto parts store. Makes retolexing a breeze! I need to restretch the grill cloth and I obviously need to clean up the lead dress cuz it looks like a rats nest in there, but its quiet ':lol:'. Any suggestions/criticisms are welcome!

[IMG:640:480]http://img.photobucket.com/albums/v416/ ... G_5098.jpg[/img]

[IMG:640:480]http://img.photobucket.com/albums/v416/ ... G_5096.jpg[/img]

[IMG:640:480]http://img.photobucket.com/albums/v416/ ... G_5101.jpg[/img]

[IMG:640:480]http://img.photobucket.com/albums/v416/ ... G_5102.jpg[/img]

[IMG:640:480]http://img.photobucket.com/albums/v416/ ... G_5103.jpg[/img]

[IMG:640:480]http://img.photobucket.com/albums/v416/ ... G_5104.jpg[/img]

Currently running it through a delta-pro 12a loaded in a HRM BM Deville I converted. Amp is still breaking in but I like it a lot so far.

[Structo]

Lots of fun aren't they?

What did you end up with on the power tubes, plate voltage wise?

[David Root]

If it's quiet, and it sounds good, no hash/fizz in OD mode, the lead dress is OK! Look at pix of real Dumbles, they don't have right angle military style lead dress either.

If the OD has some fizz I would go over each grid lead and see if they can be shortened, or at least move them around a bit..

Other than the grille cloth it looks great, especially the nice tolex job.

[zimm]

Lots of fun aren't they?

What did you end up with on the power tubes, plate voltage wise?

They are fun, but I dont know how people can hand build for a living, takes a lot of time! This one took me a lot longer than the Deville retrofit because everything was from scratch.

As far as plate voltage, its a little over 700V I think , its over my fluke meters range and I need to borrow one from work that goes higher. Preamp B+ is about 450V.

[zimm]

I was in the amp to put in the Din connector for the FS and measured the voltages:

Main B+ 672V
Screens 350V

V1a 184
v1b 189

v2a 196
v2b 204

V3 312 & 305

Dlator with stancor PS-8415 trans., choke:

Pin 1 258V
Pin 3 30 V
Pin 6 233V

If anyone thinks anything is off please let me know. I might lower the screen resistors to 470K now that Im running 5881 tubes, not sure if it's worth fixing? Thanks!

[markusw]

Looks great!
Is this Ampeg grill cloth?
May I ask how you wired the zener diodes?

Peace,

Markus

[zimm]

Thanks! Good eye, that is Ampeg cloth I had laying around from an older project. The zener schemo is in this thread:

http://ampgarage.com/forum/viewtopic.php?t=2818&start=0

I wasnt sure where I should put them, but I wound up placing them on the rectifier board, and copying the MM schemo for the rest of the board. I have a type I power transformer so wiring is a little different than the more common (?) type II. The 25K resistor is the large maroon one on the filter board. Not sure if I did this right but like I said, things are quiet.

[talbany]

If anyone thinks anything is off please let me know. I might lower the screen resistors to 470K now that Im running 5881 tubes, not sure if it's worth fixing? Thanks!

Zimm
What is it you wish to accomplish by lowering the screen resistors?... Remember the screens are already running half the plates... I think you mean 470 ohms not K.

Tony

[zimm]

470ohms, yes, sorry. I do not wish to accomplish anything in particular, just have it sound the best that it can and be "right". There are some that say there is no audible difference between 1K and 470, and others who say there certainly is. In the end Ill have to try for myself because In the end Im the only one that needs to be happy with it You guys have way more experience than me so Im always open to suggestions. I guess the main goal is to avoid spending time tweaking the amp to the point of pissing off my wife. Thanks!

[Structo]

Hehehehe, unfortunately or fortunately, however you look at it, you have probably built the most tweakable platform there is.
It's very hard to leave it alone once you start.

[talbany]

470ohms, yes, sorry. I do not wish to accomplish anything in particular, just have it sound the best that it can and be "right". There are some that say there is no audible difference between 1K and 470, and others who say there certainly is. In the end Ill have to try for myself because In the end Im the only one that needs to be happy with it :lol: You guys have way more experience than me so Im always open to suggestions. Thanks!

Zimm
I didn't mean that to sound condescending..Just thought you had a special purpose for lowering the screens that's all..
Here is what I am thinking.. Since the screens are running at a much lower voltage than the plates the need for the higher value screen resistors aka 470/1K won't buy you anything in the way of regulation (like a class AB push pull running close voltages to the plates) will.. The higher value screen resistors (in the case of the MM) will only cause the tubes to run less efficient or translated not as loud..I will say the higher value screen resistors will effect the symmetry and character of distortion as well as the crossover notch (@ high volumes) so in this respect you may like the higher values but doubt there will be an audible effect on tone (treble middle bass frequency's)..unless your EJ
As a matter of fact the choke may provide enough regulation for you to run the screens without the resistors as long as you didn't run the amp at full out for long periods of time.. It is however a good idea to put a low value resistor on the screens in case they for what ever reason begin to pull too much current you take out the resistor instead of a transformer Power or possibly choke.. BTW Dumble used 150 ohm screen resistors on his MM amps..Hope this makes some sense.. Anyone want to jump in and add something feel free!!..
Stek 5881 going on 2 years in my MM and been bounced around in the back of a semi for half that.. Still sounding good!!
Tony

[grtamp]

Hi Zimm,
excuse me but I don't understand very well from your photos, have you wired 2x100uF 450V cap in parallel with 270K drop resistor?

[zimm]

"I didn't mean that to sound condescending"


Tony, not interpreted as condescending at all, we're cool, thank you for that explanation , makes sense.

Grtamp, I have a standard filter set up, I have two 100uf caps UNDER (on the opposite side of the filter board) the two in the picture. So I have two parallel 100uf caps, in series (4 caps total). I did this because I did not have 220uf caps a la the ods101 schemo, but did have some extra 100uf so I did it this way. Does that make sense?

[Structo]

Not sure if this will help or confuse but this shows several ways of filter caps in series with balancing resistors.

The resistors help to balance out the voltage to each capacitor.
And as a added benefit they also bleed the voltage to ground when the power is turned off.

[zimm]

Thanks Tom, my set up is like the "bottom Left diagram", except I have paralleled two caps before putting them in series.