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EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes

22euu 8076 16
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How can I find a push-pull amplifier schematic or reference design for EBL21 tubes, and what output transformer impedance should I use?

An EBL21 push-pull stage can be based on the “Radio AGA Special Version for many Speakers line” reference, with a schematic/info link at oldradio.pl [#19611576] [#19611590] The original output transformer parameters are not available, and the receiver was made in very few copies, so exact transformer data is unlikely to be found [#19611656] EBL21 has very similar parameters to a 6V6, so it can work in push-pull with Ra = 9 kΩ or 10 kΩ transformers [#19613947] For less demanding use, PP transformers intended for EL84 / 6AQ5 with Ra = 8 kΩ are also suggested [#19613947]
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  • #1 19611521
    22euu
    Level 2  
    Posts: 2
    Dear colleagues,

    I'm not Polish but I'm as passionate about electronics as you are. Because I received several EBL21 tubes, I'm looking for a schematic (or links) for a tube amplifier with EBL21 used in Push-Pull mode. I have searched on the internet but I didn't find, only SE mode.

    If you can help me with a link or information I will appreciate it.

    Thank you in advance!
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    #2 19611576
    jozgo
    Level 42  
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    Radio AGA Special Version for many Speakers line.
    Attachments:
    • EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes AGA RSZ-F-8W.png (2.53 MB) You must be logged in to download this attachment.
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  • #4 19611601
    22euu
    Level 2  
    Posts: 2
    Thank you for the link. Do you have the specifications about output transformer? It must have been manufactured.
  • #5 19611624
    aksakal
    Tube devices specialist  
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    EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes
  • #6 19611656
    jozgo
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    22euu wrote:
    Do you have the specifications about output tranformer? I

    Unfortunately. I do not have the parameters of this transformer. And we are unlikely to find them. There were few copies of this radio receiver. One, maybe the only one, has my friend. I revived him. Works.
  • #7 19613947
    aaanteka
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    The EBL21 lamp has very similar parameters to the 6V6 lamp. It can easily work with PP Ra = 9kohm and Ra = 10kohm transformers.
    For less excessive applications it seems possible to use PP transformers dedicated to EL84 / 6AQ5 with Ra = 8kohm.
  • #8 19671042
    pawlik118
    Level 33  
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    EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes
  • #9 19671088
    W.P.
    Computer PSUs specialist  
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    The diagram in # 5 lacks the grid polarity in the phase inverter (pin 7).
    The diagram in # 5 lacks the grid polarity in the phase inverter (pin 7).
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  • #10 19671882
    aksakal
    Tube devices specialist  
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    Buddy, WP! Thank you for being careful and for correcting the mistake. It never occurred to me that such a solid source of information could allow for such an elementary oversight, good science for the future - Trust him, but check!
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  • #11 19671910
    cirrostrato
    Level 38  
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    W.P. wrote:
    The diagram in # 5 lacks the grid polarity in the phase inverter (pin 7).
    And you are convinced that in this system it is needed, compare a similar system https://frank.pocnet.net/sheets/046/e/ECH21.pdf, diagram on page 18. Regards.
  • #12 19672074
    jozgo
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    cirrostrato wrote:
    And you are convinced that in this system it is needed,

    Well, the net cannot "hang in the air". There must be a resistor in the grid.
  • #13 19672083
    cirrostrato
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    jozgo wrote:
    cirrostrato wrote:
    And you are convinced that in this system it is needed,

    Well, the net cannot "hang in the air". There must be a resistor in the grid.
    After all, it is between the grid and the ground with a value of zero ohms. I can assure you that there is no error in the schematic, it will work as is.
  • #14 19672261
    jozgo
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    cirrostrato wrote:
    between the grid and ground of zero ohms.

    Well, it is not. It is infinite.
    In this arrangement, the mesh should be a little (1 - 2 volts) below 1/2 ua.
    So a divisor of e.g. 1 M +10 k / 1M between Ua and mass.
    Then the grid will be approx. (-) 1.5 V to the cathode and will not 'float'.
  • #15 19673129
    aksakal
    Tube devices specialist  
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    2 classic variants of this type of phase inverter circuit. EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes EBL21 Push-Pull Tube Amplifier: Schematics, Links, and Information for EBL21 Tubes
  • #16 19673191
    cirrostrato
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    jozgo wrote:
    cirrostrato wrote:
    between the grid and ground of zero ohms.

    Well, it is not. It is infinite.
    We are talking about the same mesh (pin7) of the heptode part of ECH21 in the last photo?
  • #17 19673202
    pawlik118
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    jozgo wrote:
    cirrostrato wrote:
    you are convinced that it is needed in this system,

    Well, the net cannot "hang in the air". There must be a resistor in the grid

    I think you are talking about two different patterns.
    My schematic from ECH21 in which the heptode part works as a voltage amplifier is alive copied from the ECH21 documentation according to the link above.
    The negative voltage of the grid pin 6 will be used for voltage gain control (the lamp is a selectode).
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Topic summary

✨ The discussion revolves around the search for schematics and information regarding the EBL21 tube used in a Push-Pull amplifier configuration. A user expressed difficulty finding relevant resources, as most available schematics pertain to Single-Ended (SE) configurations. Responses included links to specific schematics and diagrams, as well as discussions on output transformer specifications, suggesting that EBL21 tubes have similar parameters to 6V6 tubes, which can work with Push-Pull transformers rated at 8kΩ to 10kΩ. There were also technical clarifications regarding grid polarity in phase inverter circuits and the necessity of resistors in the grid to prevent floating voltages.
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FAQ

TL;DR: For EBL21 push‑pull, target Ra 9–10 kΩ (8 kΩ works). "very similar parameters to the 6V6." Build or repurpose an EL84/6AQ5 transformer, verify ECH21 inverter grid reference, and expect scarce OEM AGA specs. [Elektroda, aaanteka, post #19613947]

Why it matters: DIY tube‑amp builders get transformer specs, schematics, and inverter gotchas to finish EBL21 PP projects fast.

Quick Facts

What output transformer primary impedance should I use for EBL21 push‑pull?

Use a primary of 9–10 kΩ for standard push‑pull loads. An 8 kΩ primary also works when using EL84/6AQ5 transformers. This aligns with EBL21 behaving like a 6V6 class tube in PP service. Start with 9–10 kΩ for clean headroom, then evaluate. [Elektroda, aaanteka, post #19613947]

Can I reuse an EL84/6AQ5 push‑pull transformer with EBL21?

Yes. For less demanding builds, an EL84/6AQ5 PP transformer with about 8 kΩ primary works acceptably with EBL21 pairs. Expect slightly different loading and gain compared to 9–10 kΩ primaries. Check bias and balance after installation. [Elektroda, aaanteka, post #19613947]

Are EBL21 tubes close enough to 6V6 for PP designs?

Yes. As one expert notes, they have "very similar parameters to the 6V6." This lets you borrow many 6V6 PP design assumptions, especially transformer Ra values. Still verify screen voltage and bias specifics on your build. [Elektroda, aaanteka, post #19613947]

Where can I find a real‑world EBL21 push‑pull schematic to study?

Look at the AGA Special Version radio reference shared in the thread. It points to a historical multi‑speaker set that used relevant topologies. Combine that with the posted thread schematics for guidance. [Elektroda, jozgo, post #19611590]

Do we know the specs of the original AGA output transformer?

No. Parameters weren’t documented publicly, and surviving units are rare. A member reports reviving one radio but confirms specs are unlikely to be found. Plan to design or select a modern PP transformer instead. [Elektroda, jozgo, post #19611656]

Which ECH21 pin is the heptode control grid in the inverter stage?

Pin 7 is the heptode control grid in this context. One shared schematic omitted its polarity, so double‑check your reference and add the required grid path. [Elektroda, W.P., post #19671088]

Do I need a grid‑leak or bias network on ECH21 pin 7?

Yes. Don’t leave the grid floating. Reference it with a leak or divider so the grid sits about 1–2 V below the cathode/midpoint. This stabilizes operating point and prevents drift. [Elektroda, jozgo, post #19672261]

What happens if the phase‑inverter grid is left floating?

A floating grid can wander, upsetting bias and stability. "The net cannot hang in the air"—add a grid resistor to define its potential and keep the stage predictable. This reduces noise and misbehavior. [Elektroda, jozgo, post #19672074]

Will the ECH21 datasheet phase‑inverter work as‑is?

Yes. A member compared the thread circuit to the datasheet and stated, "it will work as is." Use the datasheet’s reference as a baseline, then tweak component values for your tubes. [Elektroda, cirrostrato, post #19671910]

Where can I see classic phase‑inverter variants for comparison?

Two classic variants were posted for reference. Review them to choose a topology that matches your parts, transformer ratio, and desired gain balance. [Elektroda, aksakal, post #19673129]

How do I choose or adapt an output transformer for EBL21 push‑pull?

  1. Target a primary of 9–10 kΩ for a pair of EBL21.
  2. If repurposing iron, an 8 kΩ EL84/6AQ5 PP transformer is acceptable.
  3. Wire secondaries for your speaker load and confirm balance under signal. [Elektroda, aaanteka, post #19613947]

Can the ECH21 heptode serve as a voltage amp with controllable gain?

Yes. In the posted build, the heptode section works as a voltage amplifier. The negative voltage at grid pin 6 is used for gain control because the tube is a selectode. [Elektroda, pawlik118, post #19673202]

Has anyone confirmed a working EBL21 push‑pull radio from this thread?

Yes. One member revived a rare AGA unit and reported success—“Works.” That confirms the viability of the shared topology and approach. [Elektroda, jozgo, post #19611656]

Is there a known error in one posted schematic here?

Yes. A reviewer noted the grid polarity for the ECH21 phase inverter (pin 7) was missing in diagram #5. Add the proper grid reference before powering up. [Elektroda, W.P., post #19671088]
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