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7189 Monoblock Project

Hello:

Here is my adaptation of the S-5000, to isolate one half of the amp portion and create a monoblock. I hope to add individual trim pots to the fixed bias (for flexibility), and the UL-Triode switches.

Please ignore the bias resistor values, I don’t have that worked out yet.

7189 Mblock_R4.jpg

I am posting as well the original s-5000 schematic, with an annotation showing the extent of the extraction (in red). In transparent green, the portion I eliminated (I gather a center channel and a damping factor adjustment jumper).

Sherwood S5000 Half Annotated.JPG

In transparent blue, a line from the screen of only the bottom output tube, it appears to be a feedback connection. It seems odd that it is only applied to one of the output tubes. This connection is has me wondering if the UL-Triode switches are a bad idea.


Best regards,


D
 
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CoolOhm said:
In transparent blue, a line from the screen of only the bottom output tube, it appears to be a feedback connection. It seems odd that it is only applied to one of the output tubes. This connection is has me wondering if the UL-Triode switches are a bad idea.
Click to expand...

The cap from the bottom output tube screen is a secondary feedback line. It is only taken from the bottom tube because the input signal is single ended, so the feedback signals must also be single ended. Since the feedback is returned to the cathode, both feedback signals are in phase with the signal at the cathode, thus providing proper negative feedback to be applied.

This cap has purpose of limiting HF response slightly to make the amp more stable and/or to tune the amp's HF response. The values used are typically from say 10 pF to 100 pF and highly dependent on the output transformer HF characteristics.

I would not recommend triode/UL switching. Pick one mode or the other, and hard wire it that way. Amps that offer this type of switching are gimmicky in my opinion because it is very difficult to optimize the feedback and HF tuning circuits for both modes simultaneously. It's like trying to build two amps in one chassis--it is very difficult to optimize for both modes of operation simultaneously. Like de-tuning a race engine so it's street legal. You won't win any races but you can use it for grocery shopping. Neither use is optimized for its purpose. All my opinion of course after having built amps many years ago that offered this type of switching. I don't waste my time with it these days.

Additionally, it's just one more thing that can possibly break. Besides, you will be tempted to throw those switches while the amp is operating, which is not a wise move. I think you will be better off by making your amp designs as simple as possible. Only put in there what you absolutely need and keep the circuit focused on a single purpose.
 
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Hi Again:

I think I am getting close to a suitable power supply for these monoblocks using 7189 tubes in push-pull with a 7199 driver section...

Thinking of going with a solid state bridge and a hefty choke. Here is the arrangement and performance in listening mode:

https://bouthilletteparizeau-my.sha...dLtXeynDmAzWYBh6AM3OoSh8MGLpNu7yz51Q?e=l3PPWu

And here is what is looks like on power on:

https://bouthilletteparizeau-my.sha...BBpL2W9hHK42sBcgs2vNuDMlxV4Twr4BLJzw?e=TqFTlr

I am planning on using a simple 555 timer to delay application of B+ while the filaments heat up (10 sec or so). The transformer is about 80-90 Watts, so to keep thing simple I was not going to bother with soft start.

Is this a good arrangement for a push-pull amp? I can also do a simple LC that gets good results, but it requires quite a bit more C than the arrangement posted here.

Kind regards,

David
 
Hello and happy holidays fellow AKers!

It has been over a year since I last posted on this project... I got some cycles in over the holidays, so I reckon it's time for an update...

To recap, this is a pair of Monoblocks I am building based on the iron from a pair of old Electrohome organs, using each a pair of 7189 output tubes in class AB, ultralinear.

The chosen circuit is the Sherwood S-5000, with only the power amp portion extracted and the tube balance feature replaced with fixed bias. The B+ power supply is solid state, full wave with choke input filter (I was lucky enough to find some 150 mA, 10H Hammond chokes for a song on ebay). I built the chassis from black walnut, and some sheet aluminum. I wont cut or drill the top deck until the circuit is working properly. All parts and brackets hand-crafted by yours truly - even the clamps for the main filter caps.

Just last night I completed one of the amp boards (home-made turret board) and successfully powered up the B+ supply and the bias supply with no tubes inserted. So far so good. I am about to wire up the heaters, install the driver-splitter tube and resume my checks.

Here is photo collection of the project so far at the link below.

https://www.dropbox.com/sh/3j7to0qfr4m6h5k/AACdfiebaf7vALTCDycPlBEHa?dl=0


If there is interest I will continue posting as I progress...

Kind regards,

David
 
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