Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Porpoise Head on May 30, 2023, 02:09:48 pm
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Dear all,
I'd like to thank you for the responses I got on my coupling cap - frequency response question. Roughly 3nf was the answer :icon_biggrin:. Having determined the coupling cap value towards the dual rectifier side of things ('Persephone' in attached schematic), I'd like to ask if anyone is willing to do a quick scan of my schematic to see whether anything seems off.
The input stage and "Thalassa" channel are based on a matchless lightning. As mentioned, the "Persephone" channel is based on the dual rectifier. The output stage is an ac-coupled source follower roughly based on Merlin's chapter on these.
Any input is highly welcome!
Hans
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Just to have your schematic easily view
(https://i.imgur.com/eM8onJg.jpg)
Franco
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Thank you Franco, I'll do it that way next time!
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I spotted the first mistake: drew the 2nd relay wrong :BangHead:
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The power supply should be titled "Zeus" :icon_biggrin:
--Pete
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Are you sure about a 68k grid stopper at the input?
It may cause a fair bit of hiss, especially for the higher gain channel.
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Would you suggest a ferrite bead and dropping the resistor altogether like in the dual rectifier?
BTW good name for the power supply :icon_biggrin:
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Just reduce the value, eg 33k, 22k.
As noise is related to resistance / impedance.
Merlin suggests 10k with a cap, but have sourced low capacitance instrument cable, it sticks in the craw for me to add capacitance there :icon_biggrin:
http://www.valvewizard.co.uk/gridstopper.html
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Just reduce the value, eg 33k, 22k.
As noise is related to resistance / impedance.
Merlin suggests 10k with a cap, but have sourced low capacitance instrument cable, it sticks in the craw for me to add capacitance there :icon_biggrin:
http://www.valvewizard.co.uk/gridstopper.html (http://www.valvewizard.co.uk/gridstopper.html)
With two triodes in || you'd want a smaller grid stopper. 33K grid stopper with 2 triodes f-3dB is ≈21KHz. Single triode w/ 33K is ≈53KHz.
Use a 15K to get similar slope characteristics of that of a single triode.
Typically, I install a 22-33pF cap from grid to cathode or grid to gnd of the first stage without any issues and I play with a 6m cable. That in itself kills off nearly all of the RF leak including some of the cell phone interference.
--Pete
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Thanks for the replies,
I had been reading up on the ferrite bead discussions over on diyaudio, but it seems that there are a lot of design variables to be held in mind when using ferrite beads. So i'll probably end up with better results using a regular resistor. Have been reading Merlin's page as well. It would seem to me that with two 12ax7 triodes in parallel the total capacitance would be ~400 pf (one triode ~200 pf). Using a 10k grid stopper, one would need 1/(2piRC) = 1/(2*pi*10*10^3*(400+330)*10^-12) = 22kHz. So using a 330pf additional cap sounds sensible. Or is my assumption of ~400pf capacitance for the parallel triodes off mark?
Thanks in advance :)
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Beads may help with VHF (TV band) interference, above 30MHz.
There is no way a little bead adds enough inductance to deter AM band, 1MHz, interference.
Especially at guitar-input impedance.
Before Y2K you had a 80% chance of no radio break-through even with no resistor(etc). But working at the funky radio station studio right UNDER the tower was sure trouble. Truckstop might be OK until police or taxi came through with thumb on the mike; and then came CB where everybody sprayed 27MC all the time.
Do the resistor. 34k (two 68k) is a rough upper limit for low hiss in clubs; studio-only might go 10k or less at the risk of unexpected "Breaker-breaker goodbuddy gotyerearszon??"
Yes, today we have new crap past 100MHz; wifi, cell, badly filtered toys. A bead won't hurt anything, and just may help up there. Just an L-R bead.... actual filters with added capacitance may clobber guitar highs (because high impedance relative to most cable-entry jobs).
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Thanks PRR. It seems like you are on every audio-related forum on the web :laugh:
I'm quite interested in what the police talk about when they drive past, so I might go with 10k 330 pf and mod from there if needed.
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Last questions before I'm off ordering parts:
Does the ac coupled source follower output stage look reasonable? I tried to follow the info on the net as good as possible, but most sources were on using a mosfet as dc-coupled source follower.
Does the power supply look reasonable? I lowered the R's in the dropping chain somewhat compared to the dual rectifier since my trafo has lower secondary voltage than the trafo they use. However, first node is after the choke and the total number of nodes is quite significant before we get to V1. I made sure in-phase triodes do not share the same node.
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> interested in what the police talk about when they drive past,
Most police/emergency radio in the US has moved to higher channels and to NFM or digital. (Or to cellphone?) To save bandwidth, extend range, and reduce interference. You can't casually pick them up now.