Single channel A15Mk2

    Hello.

    I've been enjoying my M15Mk1 for years, and recently I bought an A15Mk2 kit, which I haven't started building yet.

    I love the early Fender clean-crunch sounds, but I have no use for hi-gain sounds, so I would like to build this amp as a single channel amp (channel 1). This would leave me (I think) with two spare valves, so I thought I might use them to add reverb and tremolo circuits. I have two questions.

    1. Is V3 shared between the two channels, or is V3b part of channel 2?
    2. If V3 does not intervene in channel 1, can I just connect R10 to R36 and R11 to C9, thereby "disconnecting" channel 2?

    The reason I ask is (a) so it will work and I won't electrocute myself, and (b) I calculate that I need one-and-a-half ECC83s for a reverb circuit and half a ECC83 for a tremolo circuit.

    Thanks and best regards,

    Simon

    Hi,

    V3a and V3b are both used for the phase inverter (PI) of the output stage - you can't use them.

    V1b, V2a and V2b belong exclusively to the Crunch-Channel and would be useable for other purposes.
    For a reverb driving stage the ECC83/12AX7 is a very "weak" tube, a ECC82 would be more suited.
    There are tubes existing, wich contain one system of a ECC83 and one of a ECC82 (for example ECC832 from JJ).
    How/where will you modulate the guitar signal with the tremolo LFO signal?

    The best will be you draw the complete circuit of your modified amp - otherwise a constructive discussion will not be possible.
    BTW If you don't detect the function of V3 by yourself how will you manage to compose your "own" amp?

    Regards, Bernd

    Jaichweiß (Andy Pipkin)

    Hello Bernd.

    Thanks for the answer, but I think you're looking at the M15 plans, in which the PI is indeed V3. In the A15 the PI is V4. I'm concerned with the A15.

    For the reverb and tremolo, I was considering using the "compact" Princeton-style circuits shown on Rob Robinette's "Add Reverb & Tremolo" page (sorry, not sure what the rules are for posting links here). I take note regarding the reverb driver valve.

    Of course, you are right about me not being knowledgeable enough to compose my own amp. That's why I bought a kit and want to modify it in the most "modular" way possible :D

    Best regards,

    Simon

    Damn, you are right.

    I took the first model name in your opening sentence. And your considerations are right. V3b is used as a cathode follower that doesn't amplify the signal (voltage gain ~ 1) but maks it low impedance. That is useful for the following "send" output. If you don't use the send/return jacks you could omit this stage.

    The rules for posting links of circuits seems to be very liberal here (compared to another German guitar/tube-amp forum). It depends more on the rules of the website where you link from.

    Regards
    Bernd

    Jaichweiß (Andy Pipkin)

    Aha! This looks like progress! So if I omit the FX loop I could connect R10 to R36 and R11 to C9 and I'd have a great single channel preamp?

    Thanks,

    Simon

    R10 and R36 are already connected and belong to the plate supply.
    If you connect R11 to C9 there would be C7, C9 and C22 in series which means low total capacitance and a poor frequency response.
    R11 makes only sense in common with R35 (insulation resistors for adding the signals of both channels).

    Bernd

    Jaichweiß (Andy Pipkin)

    OK. How can I connect channel 1 to the power amp, removing channel 2, assuming that I will sacrifice the FX loop in order to use V3 for other purposes?

    Best regards,

    Simon

    Hello again.

    Being, as it is, that I have only a rudimentary knowledge of this kind of thing, I am going to make a couple of deductions/assumptions regarding the A15Mk2 circuit using the comparison of the M15 and A15 circuits. Of course, I won't put anything into practice until I'm sure it's safe and workable. But perhaps I will learn something this way.

    I assume that if channel 2 doesn't exist, then R11 is not necessary. Actually, you told me that.
    I assume that if neither channel 2 nor the FX loop exist, then neither C9 nor C22 are necessary.
    I assume that the answer to my question is to anull the channel switches and connect C7 to the PI.

    Am I right?

    Best regards,

    Simon

    Hello.

    Given that I am going to eliminate channel 2 and that I am looking for more clean headroom, would it be productive to use a 5751 or 12AT7 in V1, or would this just throw the whole circuit out of balance? How about in the PI?

    Regards,

    Simon

    Hi,

    5751 is a Hi-Q Version of the 12AX7 = ECC83.
    12AT7 = ECC81 which has lower µ of 60 [100] but higher conductance 5,5 mA/V [1,6] and lower Ri 11 kOhms [62,5] than a ECCC83 [ ].
    So, the latter one would eventually get more headroom and less disortion but in total also less output.
    You have to check whether the operation point is OK - for you, because guit amps are a very subjective matter ;)

    Regards,
    Bernd

    Jaichweiß (Andy Pipkin)

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