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Old 8th March 2015, 10:14 PM   #1
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Default Two driver options for 300B push pull

Hi:

Been using a 300B PP amp for a number of years now (actually it's been in storage for a while) and considering making some upgrades and possible changes. I have two options that I am considering. Currently I am using something very similar to Option 1, but without the current sources. Using the 40 uF cap to cathode in each stage with a single bias resistor in the tail. Also been AC heated with less than 1 mV hum at the speaker, but going to go DC heating as long as the chassis is getting rewired.

Ultimately, I am trying to determine the optimal way to eliminate the first IT, if there is a better way to do it. I have summarized my thoughts below, feel free to chime in. I know it may not be to everyone's taste, with a lot of iron, but these are the two options I am currently considering. If it makes you feel any better, I am relocating the XLR input to the secondary of the input xfmr, using it as a balanced grid choke. RCA will be wired to the primary in standard fashion.

Option 1:
Benefits-
a. 150V and 300V supplies currently implemented with CCS-fed 0D3 shunt regulators, so are extremely quiet power supplies with a pretty glow.
b. First stage fully differential with a single adjustment, set and forget.
c. 46 is an excellent driver for 300B, and seems to have no issues with the IT, but is a little pricey.

Deficits-
a. First stage IT adds some phase and gain shifts at the higher frequencies. A bifilar IT may help in this regard, but testing will be at a future date. Asking a lot from an input stage.
b. 46 stage will require the extra DC heat circuitry under the hood, with possible contamination from stray magnetic fields.
c. Driver stage has 40uF cap, which would be nice to eliminate. Could put another CCS in the tail, going fully differential on the driver stage, not sure if this would be an improvement or not?
d. Negative DC supply required.

Option 2
Benefits-
a. No negative supplies needed.
b. First stage should be very linear with CCS load.
c. DC coupling exhibits no blocking distortion.
d. 6W6 has 1.6K Rp, so should be an excellent driver for 300B.
e. I like octals
f. Tubes are cheap and easily hand selected by test. If I get bored I can always swap in some overpriced 6SN7.
g. AC heating for first two stages is much simpler, but xfmrs need to be added to the signal chassis.

Deficits-
a. Not sure how stable and reliable the bias conditions will be. The CCS are adjustable, so in theory I should be able to closely dial in the 150V to produce good current match in 6W6 stage (place small R in cathodes).
b. Sad to lose the 46 DHT as a driver, but it just doesn't bias well in this topology.
c. 375V power is unregulated with over 500mV ripple. Not an issue for the first stage, shouldn't be a huge issue for the PSRR of the driver stage, but a tiny bit does seem to get through in my bench test.
d. Would like more than 150V on the plate of 6N8S, but there's only so much I can do with the existing 375V supply while retaining DC coupling.
Attached Files
File Type: pdf Option 1.pdf (12.7 KB, 172 views)
File Type: pdf Option 2.pdf (12.8 KB, 150 views)
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Old 9th March 2015, 02:13 AM   #2
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Wow, so many factors to consider in making these changes...

Quick question, are you planning on being able to drive the 300B grids positive at all?

What do triode-connected 46 curves look like? Is rp fairly low?
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Old 9th March 2015, 03:34 AM   #3
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Well, I shouldn't need to run the grids positive, but I wouldn't complain if I could. Thus far I have found that the 300B stage distorts well before the driver stage, so am happy with what I have capability-wise in the driver.

Rp around 2.4k for the 46, 1.6k for the 6W6
Attached Files
File Type: pdf 46-2.pdf (323.4 KB, 46 views)
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Old 9th March 2015, 03:58 AM   #4
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I read about regulated supplies and ccs bad for IT coupled stages. Why complicate everything, if you don't use feedback just an IT. IT provide already a good deal of filtering.

See this: a parallel 300b or single tube driven with transformers all the best from Lundhal. The only thing better I could imagine is a 3 x 300B parallel but the transformer ''gap'' would start to compromise the sound which nullify the sound quality gain obtained from a lower output impedance.

http://www.audiodesignguide.com/my/pse2.html
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Old 9th March 2015, 10:48 AM   #5
lordoff is offline lordoff  Bulgaria
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Quote:
Originally Posted by gabdx View Post
I read about regulated supplies and ccs bad for IT coupled stages. Why complicate everything...
I'm in the same "why complicate everything?",
but frequently is possible to be over-simple.
I'm curios where you read that CCS and regulated supplies (I guess you think about series-shunt regulators)
destroy sound on IT coupled Differential amplifier stages.
My listening and reading show me oposite: Lynn Olson's and Kevin Carter designs for example.

zigzagflux:
I will try Option 2. Salas SSHV2 regulator Simplistic Mosfet HV Shunt Regs maybe is good option for you.
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Old 9th March 2015, 12:20 PM   #6
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I guess power supply regulation was not a big factor for me, other than to point out when I did use it, it provided excellent results. Altering the regulation is not really an option in my mind, as long as the bias conditions allow it. Altering the regulation in any way complicates matters, a gas tube is about as simple as it gets.

The issue is the topology differences between Option 1 and Option 2. Lordoff, I have been leaning towards Option 2 for a number of years, but haven't found the perfect design that fits into my existing 375V power supply. This one is the best I have found so far.
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Old 9th March 2015, 01:16 PM   #7
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Quote:
Originally Posted by zigzagflux View Post
Hi:


Deficits-
a. First stage IT adds some phase and gain shifts at the higher frequencies. A bifilar IT may help in this regard, but testing will be at a future date. Asking a lot from an input stage.
b. 46 stage will require the extra DC heat circuitry under the hood, with possible contamination from stray magnetic fields.
A) in my experience bifilar winding is not a good option for center tapped input transformers. It can be good for SE inputs or interstage, if you look for good quality at low cost.

B) DC heater supply can be a challenge. IMHO a CLC filter is a very rewarding option providing you have room and budget for a good quality inductor
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Old 9th March 2015, 02:15 PM   #8
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Any interstage drive can drive the grids positive, which is a very good thing, imo.

I'd not skimp on the swing of the driver given the IT being there, cap coupled might be a different thing.

Two xfmrs will generally sound very different than a direct coupled two stage input/driver.

Don't count on an IT having anything like ideal response, that's the first thing.

The two stage input circuit likely will vary quite a bit in terms of subjective sound depending on the parts, tubes and operating points, as well as the way it is loaded.

I don't like cathode bias in general, Ymmv.

I think you need to build more than one amp?

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Old 9th March 2015, 03:15 PM   #9
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Quote:
Originally Posted by zigzagflux View Post
Well, I shouldn't need to run the grids positive, but I wouldn't complain if I could. Thus far I have found that the 300B stage distorts well before the driver stage, so am happy with what I have capability-wise in the driver.

Rp around 2.4k for the 46, 1.6k for the 6W6
The reason I was asking is that if you want to drive the grids of the 300Bs positive at all, you might want to avoid a CCS tail for the driver stage. Push-pull operation might be desirable rather than differential operation there. You don't want the fact that one side sees a difficult load (grid current in the output tube) to cause distortion in the other side.

Now, if you were to go with the differential driver, it may still be okay since the other tube might be cut off when the one requires grid current but you would want to work things out and make sure before you build it. Maybe you already have.
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Last edited by SpreadSpectrum; 9th March 2015 at 03:19 PM.
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Old 9th March 2015, 09:07 PM   #10
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Quote:
Originally Posted by CiroMarzio View Post
A)in my experience bifilar winding is not a good option for center tapped input transformers
Agreed, I probably did not clarify enough. Input transformer is shielded and dual bobbin, whereas the first and second stage IT will be bifilar. (Or just the driver stage if I elect to remove the first stage IT) With that in mind I am hoping the IT does an even better job than the layered winding IT's I am currently using. They certainly work up to around 18k, but tend to get pretty wobbly after that. Always looking to improve. My experience with bifilar signal output transformers has been very astounding. When you don't need a low winding/winding capacitance, they're the way to go. This will be my first go at bifilar interstage.

Quote:
Originally Posted by SpreadSpectrum View Post
The reason I was asking is that if you want to drive the grids of the 300Bs positive at all, you might want to avoid a CCS tail for the driver stage. Push-pull operation might be desirable rather than differential operation there. You don't want the fact that one side sees a difficult load (grid current in the output tube) to cause distortion in the other side.
Thanks, that's the kind of feedback I have been interested in hearing. I was not certain how a fully differential stage would respond to the load of 300B grids as we near 0V. It would be easy enough to put in the cathode resistor with a 40uF cap from B+ to cathode. Is that the optimal driver stage you are referring to?

I have not built the output stage as being fed from a differential-fed IT. It's a bit of work on a small bench to temp some of these test circuits up. The closer I get to deciding on a path before proceeding the fewer gray hairs I get.
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