Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: boodle on July 01, 2024, 09:42:49 am
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Hello all,
I am in the process of updating a new build sonically and will add a one tube reverb while I'm in there.
Schematic pdf attached. ECL84 datasheet attached. Moody GA40 reverb schematic trace attached.
4 questions:
1. On another schematic that I was using as a guide, the builder put a 22nF cap between the (treble control) output of the stack and the two 270k mixing resistors. That seem unnecessary as all DC has been blocked to the TS. Am I correct in this thinking or am I missing something?
2. The Phillips ELC84 datasheet notes that the pentode has a Ra = 3k @ 170-220V. First, I am assuming that is the load impedance, could be mistaken. Also, the pentode plate resistor will be fed at ~270V.
Am I reading this right and, as a result, should use one of the tank codes listed on the schematic, 8EB3C1B or 8FB3C1B? (Has to be a small tank due to cabinet size, want 3 springs for that sound, want the long decay so that I have max control of decay with the Dwell knob.)
3. A general question regarding the reverb circuit: Do you seen anything that I should change?
I am using a couple of schematics for a guide, one of which is a trace of the Moody GA40's ECL84 reverb. The Moody has quite a bit of extra stuff on the grid and plate of the ECL84 triode that I have simplified. I am assuming that the extra stuff is for tweaking the tone of the verb. I might need to go back and do that, but didn't want to start there. Also, I moved the reverb level control to before the triode recovery stage.
Thank you for any assistance that you have to offer!
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Anyone with thoughts?
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Anyone with thoughts?
I haven't analysed the operating point in detail but it looks like a solid circuit. Personally I would just run the driver in triode mode for simplicity (e.g. Traynor), and maybe increase the 2.7k load resistor a bit to save some current. You want the highest impedance tank you can get.
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Merlin,
Wonderful! Just what I was looking for. I appreciate it.
In terms of raising the 2k7, you thinking 10k-ish or do you have some other recommended starting value?
Do you suggest I have any resistance between grid 2 (pin 9) and plate (pin 6) or just tie them together for triode mode?
Proposed updated schematic attached.
Tim
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Wouldn't you need a mixing circuit?
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Williamblake,
Thanks for looking at it!
I believe the two 270k resistors just after the tone stack are the mixers. No?
Tim
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In terms of raising the 2k7, you thinking 10k-ish or do you have some other recommended starting value?
Do you suggest I have any resistance between grid 2 (pin 9) and plate (pin 6) or just tie them together for triode mode?
Yes 10k would be fine, and just tie the screen straight to the anode. Traynor for reference (wonderful reverb on those things!): https://el34world.com/charts/Schematics/files/Traynor/Traynor_ygl_3_ygl_3a_mk_iii.pdf (https://el34world.com/charts/Schematics/files/Traynor/Traynor_reverbmaster_yrm1.pdf)
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The output signal will probably be much weaker than the signal coming from the first gain stage of your amp.
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Merlin,
Thank you for the link and advice, very helpful.
Williamblake,
If I am understanding correctly, I may need to reduce the 270k mix resistor coming from the reverb side to get the max verb amount I might want. Reading you correctly?
Or are you suggesting that I rebias the triode part of the ECL84 for more output?
Tim
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If I am understanding correctly, I may need to reduce the 270k mix resistor coming from the reverb side to get the max verb amount I might want. Reading you correctly?
Or are you suggesting that I rebias the triode part of the ECL84 for more output?
Yes you can always trim the mix resistors on test, to get the depth you need. Rebiasing won't have much effect on the recovery gain.
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Merlin,
You are a gold mine! thanks