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 Post subject: How to set Bias on HH Scott 299B and plate voltage too high?
PostPosted: Mar Sun 17, 2019 8:19 pm 
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I'm puzzled on how to set the bias in this amplifier. I spent many hours recapping this unit and found that the bias circuit did not match the schematic. The schematic for Channel A is shown below. My unit differs in that I do not have the meter test terminal or R60 (the 18 ohm from cathode to ground). Both cathodes are connected directly to ground. Another difference is that the center tap of R7 (bias control) is also connected to the DC Balance control for Channel B (not shown). That is to say, there is only one bias control serving two channels. The original schematic shows an additional bias pot (R9) for channel B.

I adjusted the balance pots for equal voltages on the control grids of both tubes. Not sure if that the right thing to do. I don't think so but I did get pretty darn close to -15V.

I have a service bulletin for the 299B (not easilly found online). I in there is states to adjust the bias as follows: "Disconnect the two straps on the meter terminal. Connect a milliammeter (100-120 ma. range) between the center terminal and the Channel A terminal, and repeat the adjustment, with the Channel B bias pot. Reconnect the two straps." Well, obviously I can't do this since I don't have the 18 ohm reistors or the test terminal strip.

The service bulletin also has a procedure for the D.C. Balance adjustment. It's to much to type out but is the same as for the 222b model described here:
https://www.thetubestore.com/lib/thetubestore/schematics/HH%20Scott/HH-Scott-222-b-service-bulletin-Schematic.pdf
I can do this once I figure out how to bias this thing properly.

Also, there is an AC Balance procedure in the same document. In there, they talk about feeding into an adjustable 400 cycle null circuit and that is simple to build. But you have to write the engineering department to get the circuit. Might anyone know what this circuit is?

So I played this amp last night for about an hour or two and then after a while, I noticed that channel A became distorted. I swapped two output tubes between Channel A and B and the distortion followed the tube. I'm not too happy about this but it led me down the path of digging out the RCA tube manual. The plate voltages in the schematic shows 420 volts which is pretty much what I have. However, the RCA Tube manual shows 375-400V max. plate voltage for class AB. Why would HH Scott design beyond the ratings? What am I missing here? All the tubes in this amp, I believe, are original as they have the name HH Scott printed on them. I'm wondering if I damaged an output tube due to this.

I know this is a long post but thanks for any help you folks may be able to offer.
The full schematic can be found here although the biasing scheme did not match my 299B.
https://www.thetubestore.com/lib/thetubestore/schematics/HH%20Scott/HH-Scott-299-B-Schematic.pdf
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Dan


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 17, 2019 9:14 pm 
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Best to install four 10 Ohm resistors, one in series with each cathode to ground. 1% resistors would be best. Then you can adjust bias and balance properly. I would try for 25 mA per tube (250 mV across each resistor). 30 mA would be right at the dissipation limit for the 7189 (.03 x 420 = 12.6W). The grid voltage doesn't matter - the current does. You may have one that's gassy or has grid leakage if it won't hold at 25 mA.

As for voltage - I have a receiver that runs about 450V on 7189s - you can usually get way with exceeding one tube spec if you stay below the others. The 400V rating is "design center" - "design maximum" would be 440V, "absolute maximum" would be 480V

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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 17, 2019 9:38 pm 
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I did buy 2 - 18 ohm resistors at 1% in anticipation of installing them per the schematic. The parts list listed them as 1/2 watt. That's a bit too low for 25ma. But maybe I'll try them at a lower current and see if that clears up the distortion until I can get 4 - 10 ohm resistors. I assume you leave them in permanently afterwards, no? I guess that's why you say the grid voltage doesn't matter?

Another thing I'm looking into is I found almost double the voltage on pin 3 of V3 (the previous stage). It should be 60v and I have something like 110. And in fact, R118's voltage drop indicates that resistor is running about 1W and it is a 1W resistor. But I'm not seeing anything wrong down the line. Guess I'll have to start disconnecting things.


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 17, 2019 10:05 pm 
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Are you sure that you have a 299B? The second generation 299 looks a lot like the 299B. Could someone have changed the faceplate in the past and you actually have a 299 second generation?

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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 17, 2019 10:08 pm 
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This is a second generation 299:


Attachments:
299.jpg
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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Tue 19, 2019 11:57 pm 
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There is no evidence of any work being done to this unit so I kind of doubt the front panel was changed. It also came with the cabinet. I acquired it in the early 1990's from someone who was going to throw it out. It sure does resemble the picture you posted except for vent holes on the top of the chassis and the Neon lamps. My neon lamps are flat vs. the cone shape in your picture. The bias supply potentiometer circuit does match one of two 299 (non-B) schematics I have where one uses a 10uf cap on the bias pot center tap. But I can say for certainty that the bias resistor values around the balance pot and output tubes and grid are a pair of 220k, 150k and 1k ( 1k to grid) as shown in the 299B schematic. Maybe this is an early 299B?

The bias
Here are a few pictures:

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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Fri 22, 2019 12:52 am 
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Ok so I put a couple 18 ohm resistors in the cathode circuit of the 7189's. On one tube I got about 17ma with the variac set to 110. (I'm not too concerned about the bias voltage at the moment.) On the other, I got zip! Looks like I confirmed a bad tube. I swapped both and the results followed the tube. Plate voltage around 400V and grid about -15V. The suppressor grid circuit I still need to figure out as my schematic is deviating from the unit but the grid is about 106V. From what I can tell, all 4 tubes combined are drawing about 1ma through the suppressor. Does that sound about right? If I understand correctly, the suppressor will pass some current but it shouldn't be much.

What output tubes do you guys use in these amps? Will the EL84, 6BQ5's that I see online for replacements work even though they have a lower maximum plate voltage? Or would I need to reduce the plate voltage or maybe turn down the bias to use them?

Thanks,
Dan


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Fri 22, 2019 2:49 pm 
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The Russian ones are the only affordable 7189s - 6P14EV, also sold as EL84M. SOME vintage 6BQ5s may take the voltage, most 6BQ5s won't.

I assume you mean "screen" grid - suppressor is internally connected to the cathode. If screen is really at 100V, current will be low - should be 360V. Something else may be loading the supply.

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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sat 23, 2019 2:07 am 
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That's right. I meant the screen grid. I was tired and in a rush last night. In fact, the screen grid voltage I was talking about is on the previous stage, the 6BL8. Not the 7981. I have to take a step back and analyze the differences I found between the schematic and my unit. There are a few different resistor values and whatnot in the PS and the 7981 bias is different. I'm a little confused as to why the schematic shows 60V on the screen grid for the 6BL8's with a plate voltage of 35. I would think the screen voltage should be a fraction of the plate voltage.

I was looking at those Russian tubes online today. Looks like that's probably the best choice. Hopefully I can find a reputable seller.


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sat 23, 2019 1:24 pm 
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I have purchased and had good luck with the Soviet tubes that Tom mentioned. Mine are all dated before the collapse of the Soviet Union so they are truly from the CCCP and NOS.

I looked at one of my 299B amplifiers and it is a little different than yours. I will try to post a picture later today. Your 299B does appear (to me) to be very close in appearance to the second edition 299. As you probably know, there were many, many mid-stream changes made by Scott during the production run of the 222/299 line.

Some things that support your amplifier being a 299B is the obvious faceplate. Another minor difference is the 299 knobs were different than the 299B. Knobs on the 299B had a groove machined for the "pointer" whereas the 299 models had a depression.

More to come...

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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 24, 2019 12:12 am 
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Is there any difference between the 6P14P-EV and the 6P14P-EB? from what I can tell, they're both the same. A data sheet I found shows the following

6P14P, 6P14P-V, 6P14P-EV

and directly beneath it:

(P14n, 6P14P-B, 6P14P-EB)

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Mine are all dated before the collapse of the Soviet Union so they are truly from the CCCP and NOS.

Don, are you talking pre-1991? Did they make them later than this?


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 24, 2019 12:23 am 
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tunerman wrote:
Is there any difference between the 6P14P-EV and the 6P14P-EB? from what I can tell, they're both the same. A data sheet I found shows the following

6P14P, 6P14P-V, 6P14P-EV

and directly beneath it:

(P14n, 6P14P-B, 6P14P-EB)

Quote:
Mine are all dated before the collapse of the Soviet Union so they are truly from the CCCP and NOS.

Don, are you talking pre-1991? Did they make them later than this?

I am talking ~ 1979 - 1984.

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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 24, 2019 1:43 am 
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I just put an order in for 4 1979 vintage 6P14P's. We'll see what happens. I'm leery of ordering from different countries. It's bad enough ordering from within the states.

I purchased two matched pairs rather than a quad matched set and the P version to save a few bucks. If I get this thing working correctly and I like the way it sounds I'll consider buying another set for backup. When I initially tried it out, it seemed to be working fine. After playing an album or two, that's when I started noticing the distortion. So I think it was OK initially. I found the amp to be rather bright. I tend to like it that way normally but this was really bright compared to a solid state Kenwood I rebuilt a few years back and have been using as my main system.

Anyway, back to the Scott, I still don't have an understanding as to why the 6BL8 (V3 in the schematic) shows 35V plate voltatge and a screen grid voltage of 60V. Looking at the 299 schematic, it also shows a higher screen voltage than the screen grid (65/110). Still looking for thoughts on this theory. My understanding is that the plate should be higher than the screen grid but I suspect there is a reason for this type of design.


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 24, 2019 4:31 am 
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tunerman wrote:
...
Anyway, back to the Scott, I still don't have an understanding as to why the 6BL8 (V3 in the schematic) shows 35V plate voltatge and a screen grid voltage of 60V. Looking at the 299 schematic, it also shows a higher screen voltage than the screen grid (65/110). Still looking for thoughts on this theory. My understanding is that the plate should be higher than the screen grid but I suspect there is a reason for this type of design.
You're probably thinking of power pentodes. This one isn't a power pentode so we don't care about maximizing power out. We want gain. Gain is gm X Ra. We want Ra to be as large as possible and that will drop a lot of voltage.


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Sun 24, 2019 2:53 pm 
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This morning I read through the RCA Tube manual. It states, "In fact, the plate voltage may be as low as, or lower than, the screen-grid voltage without serious loss in signal-gain capability." So I guess that answers that. Thanks Filpperhome for making me dig a little deeper. The other tube theory book I was reading didn't mention this (or I missed it). Now if I recall, last night I did find that the plate voltage was higher than the screen voltage so I'll have keep digging.


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Mon 25, 2019 10:51 pm 
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Looks like you've already ordered the power tubes, but if you don't want to wait for overseas shipping or there's a problem, I'd recommend Jim McShane for tubes. I bought a set of Russian tubes for my 299 from him, and by chance, he lives very close to me. He invited me over to pick them up from his house and let me listen to his system. He's a really nice guy, and well known tube guru. http://www.mcshanedesign.net/tubes.htm

Dave


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 Post subject: Re: How to set Bias on HH Scott 299B and plate voltage too h
PostPosted: Mar Fri 29, 2019 1:36 pm 
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Yes, I did already make the order. Thanks for the link Dave. I checked it out and recognized the site. I came across it when searching for the tubes but decided against those since I found a set for half the price. Although, the set I bought are 6P14P rather than the 6P14P-EV. Looks like I messed up. For some reason I thought the plate voltage between these two versions was the same and the only difference was the life rating. Now I'm seeing the 6P14P's have a plate voltage rating of 400V rather than 500V. This unit has a 420 Volt plate supply. I've been running this at lower power using a variac so I'm not too worried. When I get everything sorted out, I'll decide if I was to alter the plate voltage and bias for safety or pick up a set of EV's. I can live with a few less Watts.


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