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Major modifications of a Bogen AP-35
Major modifications of a Bogen AP-35
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Old 26th March 2010, 06:20 AM   #1
Binaural is offline Binaural  United States
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Default Major modifications of a Bogen AP-35

This amp could be compared to a sketch book for an artist or a journal for a reporter. It's my first tube amp and have been tinkering with it and learning as much as I could about tube circuit design and how the stuff works (or supposed to) So much smoke has come from this unit, but has also taught me so much! Below are the original schematic and then a rough sketch of what's in (or hanging off of) the amp currently.

Click the image to open in full size.

Click the image to open in full size.

As stated in the schematic, it's definitely still a work in progress and probably won't ever be completed unless it winds up as parts for other amps or a "catastrophic failure" happens.

I'd also like to get some feedback, what do you think? What would you do differently? What would you add/subtract from it? It's currently driving a pair of Yamaha NS-10's and a Yamaha active sub matched to the NS-10's. It's fed by a Panasonic SV-3800 as a D/A converter for my Pro Tools LE rig I use for mixing.

Also tonight I've recorded some thoughts as to the sound of different 9V batteries used in the bias sections.

I have tried 4 different brands and 2 types of batteries in here and while I initially didn't think there would be an audible difference, boy was I wrong.

The 4 kinds tried were: Energizer Alkaline, Duracell Alkaline, Ray-O-Vac Alkaline and "Heavy Duty" Zinc Chloride.

I was completely taken aback when I found I strongly favored the cheap zinc chloride! These bad boys were $.97 for 2 compared to $5.80 for 2 Duracells and 2 Energizers!!!

Is the difference in sound due to the difference in the materials used? Impedence? What is going on here?

Finally, the original input/driver section was cathode/auto bias (Rk=4k, 22uf bypass cap) but found I preferred fixed bias for the difference in the low end and separation of the individual instruments in the soundstage.

A lot of the knowledge I used to do this came from this forum and the guru's who generously cast out their priceless wisdom to complete amateurs such as myself. I'd love to hear your comments. I look forward to learning from you.

Joe
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Old 27th March 2010, 09:16 PM   #2
Binaural is offline Binaural  United States
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Just realized a mistake with my schematic. The 4 cathode resistors on the 6l6's are drawn in parallel on the schematic but the should be in series. So instead of having a total of 100ohms in the cathode, it should be 400ohms. Whoops!

Joe
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Old 28th March 2010, 01:13 PM   #3
ray_moth is offline ray_moth  Indonesia
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Don't have time now to do a detailed analysis but a few observations (ignoring the Bogen schematic, which I can't read):

* the OP tubes must have grid resistors to ground or the bias will be unstable

* it is important also to provide stopper resistors for the control and screen grids of the OP tubes, to prevent parasitic oscillations

* the cathode resistors of the OP tubes should be bypassed with, say, 470uF caps

* you should get better performance if your drivers are configured as a differential amp; forget battery bias

* presumably a phase-splitter will be added at some stage?

PS you could do worse than buying and reading Morgan Jones's Valve Amplifiers. It will help to fill the gaps in your knowledge and understanding.

Last edited by ray_moth; 28th March 2010 at 01:17 PM.
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Old 29th March 2010, 06:26 PM   #4
Binaural is offline Binaural  United States
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Hey Ray,
You know what? I completely left those off of the schematic by accident! Rookie mistake. Coming out of the parallel coupling capacitors there is a 470k resistor to ground for both 6L6's as well as 1k grid stoppers, 1 for each 6L6.

There isn't any stopper resistors for the screen, both the screen and plate are directly connected to the transformer. What is an optimal value for the screen stopper resistor? How would the absence of this resistor cause the circuit to oscillate? I'm not trying to say you're wrong or anything like that, I'm just trying to wrap my head around exactly what's going on in the output stage.

I'm not sure what you mean by getting better performance by configuring the driver stage as a differential amp and to forget battery bias. A phase splitter in this circumstance is not needed because the amp is being fed by a balanced line. The original circuit had an unbalanced input that fed a cathodyne phase splitter. It didn't make any sense to me to have to short low to ground to convert a balanced input to unbalanced only to pass through a phase splitter to balance the signal again! Thus, you have the circuit shown here.

Sorry about the poor quality of the Bogen schematic. Here's a link to a (hopefully) easier to read copy.

http://www.abdclub.com/Audio/BogenAP30.jpg

One thing I've never been able to find out is what exactly the difference is between "differential" and "balanced?" Are they one in the same or are there differences? What would be needed to convert this driver stage to differential? Also why do you suggest loosing the fixed bias in the driver stage?

I've read (sorry I cannot recall where) but in class A power stages, a bypass cap in the cathode is not needed. I cannot recall exactly why but something about the constant draw of current. Also if there was one needed, wouldn't bass reproduction be affected? The bass in this amp is very quick and punchy. Not a bit of sluggishness or "flabbyness."

Thanks for taking the time to reply to me. I'm looking forward to your detailed analysis of the circuit, now that you have the left out portions! All i can say is the circuit as it is currently is TERRIFIC sounding. Trying not to sound cocky but my line of work (well my hobby that is currently providing me income) exposes me to professional sound reproduction systems used for critical listening, and this amp I feel could hold up very well in one of these facilities. Of course I'm a bit biased in this case, but it is definitely the best sounding amp I have owned so far.

Again thanks and I look forward to your response.

Joe
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Old 30th March 2010, 01:20 AM   #5
ray_moth is offline ray_moth  Indonesia
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Hi Joe, thanks for the link to the Bogen AP30. The differences between your amp and the Bogen are so great that I don't see much point in referring to it! You are using higher power OP tubes in UL, whereas the Bogen uses 6V6-equivalent 7408s in pentode-mode.

Thanks, also, for clearing up the issue regarding the missing grid resistors and stoppers, and I'm glad it's working really well for you.

The purpose of adding screen-stoppers is twofold: firstly, to prevent any tendency to parasitic oscillation; secondly, to lower slightly the screen voltage, to ensure it stays below the plate voltage. They're mostly precautionary and a value of 470 to 1k should be OK, although admittedly many UL designs omit them.

If you configure the drivers as a differential amp, it means that you use a shared cathode resistor ("tail"), as in the Williamson. This is generally better than separate balanced amplifying stages which you have now. It gives very good balance and low distortion. The bias will be provided by the tail resistor, so there will be no need of bias batteries.

In the OP stage, separate bias resistors should be bypassed so as not to increase the OP impedance, which would result in loss of power. An alternative arrangement would be to use a single shared cathode resistor of half the resistance of one of the single resistors you use now (i.e. 200 ohms).

Last edited by ray_moth; 30th March 2010 at 01:30 AM.
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Old 1st April 2010, 08:50 PM   #6
Binaural is offline Binaural  United States
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Ok there were just too many mistakes on the last drawing, so I made a new one. I love it, you can't get anything past you guys! Let's see how error free this one is.

Click the image to open in full size.
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Old 1st April 2010, 09:12 PM   #7
Binaural is offline Binaural  United States
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Ray,

With the shared cathode resistors in the driver stage, do these need to be bypassed as well?
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Old 2nd April 2010, 07:37 AM   #8
ray_moth is offline ray_moth  Indonesia
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No, don't bypass them or they won't be differential. Suggest you look for a Williamson circuit to see what it looks like.
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Old 2nd April 2010, 08:07 AM   #9
kavermei is offline kavermei  Belgium
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Hi,

that's a very original schematic!

With the battery grid bias in the input stage, you need to make sure that the source you hook it up to (preamp, CD player, whatever) has output capacitors or transformers. Otherwise you will be forcing -9V upon its output stage, which it might not like.

In the schematic the filament supply for the #26 appears to be shorted out. You probably forgot 2 resistors from the filament pins to ground?

Your change in coupling caps from 9nF to 109nF is rather drastic... (just a thought)

The shared cathode resistor in the output stage is sth. I haven't seen very often, but it works. However, in class AB operation if one of the 6L6s cuts off, you will get degenerative feedback. That's because at that point, the increasing current in one tube is not cancelled anymore, in the cathode resistor, by the decrease in current in the other tube. This'll cause some compression of the signal peaks, depending on how deep in class AB you are biasing.

Enjoy it!

Kenneth
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Last edited by kavermei; 2nd April 2010 at 08:10 AM.
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Old 2nd April 2010, 02:29 PM   #10
ray_moth is offline ray_moth  Indonesia
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Kenneth, the shared cathode resistor in the OP stage is only there because Binaural mentioned Class A. With Class AB, separate bypassed cathode resistors would be required. Binaural's original schematic showed separate unbypassed resistors, which is not a good idea.
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