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EL84/6BQ5 Ultra-Linear Push-Pull Recomendation

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I have some old parts in my stock and would like to make something usefull with then.

Here is what I have:
350-CT-350, 6,3V, 5V Power transformer
8K, 10W PP w/ UL taps Output transformer
RCA 5R4 Rectifier
RCA 8BQ5 (smoked glass) Power Pentodes
For de pre-amp/phase-splitter I have one Siemens E88CC, a lot of 12AX7, EF86, 12AU7 and 6SN7.

I think that I can do something very good with this parts, but I'm not convinced with design follow to build this amplifier.
My aim is 10W class A and 4 tubes, maximum 5. This is gonna be a mono amplifier.

Which design do you recommed?

Thanks in advance.
 

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gorgeous EL84s, I have a pair of old Magnavox el84a, and there pretty cool.

Personally, look into the Red Light Disctrict, it pops up on the forum about once a week, I want to build it, but it'll have to wait a while, but it looks like a solid design.

you can do the simple el84 seen on http://www.diyparadise.com/simpleel84.html
although it is self splitting, and some people hate that. I have the line stage out of it, and it works as a great pre-amp
 
Ex-Moderator
Joined 2004
That article is very interesting! I am a little puzzled as to how a triode (Figs 3 and 4) can be used as a CCS. With a pentode, you have the screen at a constant voltage, which guarantees that the plate current won't change significantly as the plate voltage fluctuates. A triode gives no such guarantee, since its current is strongly influenced by the plate voltage.
 
ray_moth said:
That article is very interesting! I am a little puzzled as to how a triode (Figs 3 and 4) can be used as a CCS. With a pentode, you have the screen at a constant voltage, which guarantees that the plate current won't change significantly as the plate voltage fluctuates. A triode gives no such guarantee, since its current is strongly influenced by the plate voltage.

Ah yes, but it's all in the cathode resistor! That resistor provides feedback to the cathode that makes it all work. Viewed from the plate, the triode CCS’s dynamic resistance is (mu+1)*Rk + rp. In figure 4, Rk is 47K. The CCS resistance would be about 1.6 mega-ohms, which is not bad. A pentode can do even better, but there are diminishing returns. Also, you often see pentode CCSs that actually are wired as triodes, perhaps inadvertently. Since the cathode voltage of the pentode CCS is NOT grounded as it usually is in a common-cathode amplifier, the screen must float along with the cathode, and should NOT be tied to a supply if you want pentode behavior. This usually requires a capacitor from cathode to screen with a resistor supplying screen current that is connected to a more positive voltage. With the cap, the CCS won’t behave as such down to “DC”. A floating DC cathode-to-screen supply can be made, but it gets pretty complicated, pretty fast. If Rk can be made fairly large, and a moderately high-mu triode is selected, the “mu+1” multiplier of a triode CCS can drive the resistance up high enough for most applications, particularly for LTP "tail" duty.
 
Brian Beck said:


Ah yes, but it's all in the cathode resistor! That resistor provides feedback to the cathode that makes it all work. Viewed from the plate, the triode CCS’s dynamic resistance is (mu+1)*Rk + rp. In figure 4, Rk is 47K. The CCS resistance would be about 1.6 mega-ohms, which is not bad. A pentode can do even better, but there are diminishing returns. Also, you often see pentode CCSs that actually are wired as triodes, perhaps inadvertently. Since the cathode voltage of the pentode CCS is NOT grounded as it usually is in a common-cathode amplifier, the screen must float along with the cathode, and should NOT be tied to a supply if you want pentode behavior. This usually requires a capacitor from cathode to screen with a resistor supplying screen current that is connected to a more positive voltage. With the cap, the CCS won’t behave as such down to “DC”. A floating DC cathode-to-screen supply can be made, but it gets pretty complicated, pretty fast. If Rk can be made fairly large, and a moderately high-mu triode is selected, the “mu+1” multiplier of a triode CCS can drive the resistance up high enough for most applications, particularly for LTP "tail" duty.


Brian,

What is an appropriate cap. value to tie g2 and cathode of a pentode together? Sticking to the "usual" down to 5 Hz. seems reasonable. "Dead reckoning" indicates that a Xicon 1429-6474, 470 nF./630 WVDC MPP part could be OK, but a more rigorous discussion would be welcome.

FWIW, I recently suggested using the pentode section of a 6U8, here, in the current sink role to load a cathode follower.
 
diegot said:
I have some old parts in my stock and would like to make something usefull with then.

Here is what I have:
350-CT-350, 6,3V, 5V Power transformer
8K, 10W PP w/ UL taps Output transformer
RCA 5R4 Rectifier
RCA 8BQ5 (smoked glass) Power Pentodes
For de pre-amp/phase-splitter I have one Siemens E88CC, a lot of 12AX7, EF86, 12AU7 and 6SN7.

I think that I can do something very good with this parts, but I'm not convinced with design follow to build this amplifier.
My aim is 10W class A and 4 tubes, maximum 5. This is gonna be a mono amplifier.

Which design do you recommed?

Thanks in advance.


You did say 8BQ5. Your biggest problem is going to be powering the 600 mA./8.0 V. heaters, whatever signal topology you settle on.
8BQ5 Data Sheet

10 W. from a PP pair of Class "A" 'BQ5s set up in pentode mode is a very realistic expectation. While I INTENSELY dislike the 12AX7 as a driver, it will be OK feeding pentode mode "finals". The basic signal topology of "El Cheapo" using a 'X7 as the splitter/driver will work. If you can scrounge up a 12AT7, you will be BETTER off.

You could get double duty from the negative supply. A LM337 adjustable negative regulator can be set up to O/P a steady 600 mA. Wire the 8BQ5 heaters in series and feed them 600 mA. of DC. 24 VAC/2.0 A. bridge rectified and 'lytic filtered, will feed the LM337 and an additional RC section for the splitter/driver.

Finally, don't forget to regulate 'BQ5 g2 B+ for lowest possible open loop distortion.
 
Eli Duttman said:
Brian,
What is an appropriate cap. value to tie g2 and cathode of a pentode together? Sticking to the "usual" down to 5 Hz. seems reasonable. "Dead reckoning" indicates that a Xicon 1429-6474, 470 nF./630 WVDC MPP part could be OK, but a more rigorous discussion would be welcome.

Eli,

That’s a great question. I don’t have the analysis at hand. Nor do I recall seeing a rigorous treatment anywhere. I’m sure someone has grunted through it; I just haven’t stumbled across it. Hopefully someone else will have the exact equations to post here. Exact equations will involve some rather tedious math because of the screen’s involvement and because of the little feedback loop that’s formed by the cap. If I have time (ha!) maybe I can grind through the analysis. I’d use a technique using the “hybrid admittance matrix”. But it might get unwieldy on me. They used to teach this analysis technique in EE colleges; I don’t know if they still do. But don’t hold your breath waiting on me.

Anyway, this might be better solved by breadboarding and testing, or by a simulation. Be aware, though, that most simulation models of pentodes don’t treat the screen with sufficient complexity or fidelity. The screen has its own transconductance, and its own resistance, like a plate resistance. But a sim might get you in the ballpark, at least. My gut feeling is that you’ve picked a too-small capacitor. Most designs would uses capacitors in the several-microfarad range.

I can tell you from inspection that the output Z as a function of frequency would probably have a stepped shape when plotted log-log. See my little sketch. The goal is to push both f1 and f2 to below the audio range. However, the CCS doesn’t disintegrate below f1; it becomes a triode-connected-pentode CCS with a degenerated screen. The overall circuit’s response to this reduced output Z may only be slight.
 

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d2134 said:


Thanks d2134 for the article. Very interesting one.

As I can't follow a design as it is published here is my thoughts:
There is too much heat generated by the cathode resistors of the output tubes. I suggest to use a lower B+ supply with a E88CC working in a lower voltage. This way the DC voltage at the output grids are lower so is possible to use a lower B+ and less heat wasted.

But the main reason for this mod is the use of a tube rectifer, because is hard to get 390V from a 350+350 xfmr, a tube rectifier and a choke (PSUD2 sims). I prefer heat wasted at the tubes instead of resistors.

I'm afraid of the gain in this first stage using the E88CC, 700mV for full output is the minimum required, I can get rid of the feedback but in this design is not a good idea.

What do you think about my sugestions for modding the design?

Sorry if I made some mistakes in my writing.

Regards
Diego
 
Re: Re: EL84/6BQ5 Ultra-Linear Push-Pull Recomendation

Eli Duttman said:



You did say 8BQ5. Your biggest problem is going to be powering the 600 mA./8.0 V. heaters, whatever signal topology you settle on.
8BQ5 Data Sheet

10 W. from a PP pair of Class "A" 'BQ5s set up in pentode mode is a very realistic expectation. While I INTENSELY dislike the 12AX7 as a driver, it will be OK feeding pentode mode "finals". The basic signal topology of "El Cheapo" using a 'X7 as the splitter/driver will work. If you can scrounge up a 12AT7, you will be BETTER off.

You could get double duty from the negative supply. A LM337 adjustable negative regulator can be set up to O/P a steady 600 mA. Wire the 8BQ5 heaters in series and feed them 600 mA. of DC. 24 VAC/2.0 A. bridge rectified and 'lytic filtered, will feed the LM337 and an additional RC section for the splitter/driver.

Finally, don't forget to regulate 'BQ5 g2 B+ for lowest possible open loop distortion.


Hi Eli.
I made some tests and with 6.3V the 8BQ5 work great, but didn't made some serious meassurements. If I would have any problems, in a local store they have some old xfmr that converts 6.3V to 8V. No problems here.

About the first stage tube, the 12AX7 isn't my first choice, I have two 12BH7, 12AT7 and another ones. My fist choice is the E88CC.

I'll not regulate the g2 because I want a UL OP stage.
Thanks for your reply.

Regards,
Diego
 
Re: Re: Re: EL84/6BQ5 Ultra-Linear Push-Pull Recomendation

diegot said:



Hi Eli.
I made some tests and with 6.3V the 8BQ5 work great, but didn't made some serious meassurements. If I would have any problems, in a local store they have some old xfmr that converts 6.3V to 8V. No problems here.

About the first stage tube, the 12AX7 isn't my first choice, I have two 12BH7, 12AT7 and another ones. My fist choice is the E88CC.

I'll not regulate the g2 because I want a UL OP stage.
Thanks for your reply.

Regards,
Diego


Diego,

I'm glad you have a solution to the 'BQ5 heater problem.

You will do just fine with UL wired "finals". However, you are going to compromise between pure Class "A" operation and a 10 W. O/P. 40% efficiency is about right for Class "A" pentode. Class "A" UL will be somewhat less efficient. It's unlikely you'll miss anything with 8-9 W. of O/P. However, if 10 W. of O/P is a must to you, use a "shallow" Class "A" operating point. The 'BQ5s will run in Class "A", except when peak power is demanded. Then, the "finals" will cross into Class "B" territory.

A Dyna style circuit, with a "concertina" phase splitter DC coupled to the voltage gain block should work using your 6922.
 
diegot said:



Thanks d2134 for the article. Very interesting one.

As I can't follow a design as it is published here is my thoughts:
There is too much heat generated by the cathode resistors of the output tubes. I suggest to use a lower B+ supply with a E88CC working in a lower voltage. This way the DC voltage at the output grids are lower so is possible to use a lower B+ and less heat wasted.

But the main reason for this mod is the use of a tube rectifer, because is hard to get 390V from a 350+350 xfmr, a tube rectifier and a choke (PSUD2 sims). I prefer heat wasted at the tubes instead of resistors.

I'm afraid of the gain in this first stage using the E88CC, 700mV for full output is the minimum required, I can get rid of the feedback but in this design is not a good idea.

What do you think about my sugestions for modding the design?

Sorry if I made some mistakes in my writing.

Regards
Diego

If you don't want to burn off B+ in the cathode resistors, you could cap couple the drivers to the output tubes and reduce the cathode resistors accordingly. Better yet, cap couple and use fixed bias on the output tubes - you already have a negative supply with this design. But now you've got a much more conventional amp and many designs already exist with values already assigned.

Sheldon
 
Self recommendation is no recommendation at all - BUT my Baby Huey mentioned above by Sheldon has a seriously gorgeous sound as the balance shunt feedback allows you to opperate with very little or even no global feedback. Lovely live sound with huge stereo imaging.
If you don't like the look of the Baby Huey I would recommend Stuarts (SY) Red Light District Amp.

HINT: for powering the 8BQ5 Heaters - Check if you have a 6.3V winding at 1.4 times the AC current you need. If so rectify it with a bridge rectifier and 4700uF to 10,000uF filter capacitor and you will end up with 8V DC - just feed the heaters with that.

Cheers,
Ian
 
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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.