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September 28, 2020, 04:42:32 PM

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11
Tubes and Hybrids / Re: 12 v tube preamp
« Last post by flester on September 25, 2020, 07:47:48 AM »
Well I built a Valvecaster running on 12 v with a 12AU7.  Did I enjoy building it? Yes. Do I like how it looks and sounds? Yes. Did I make the tube  unnecessarily visible to look cool? Yes. Was it worth the trouble? Yes for me. Will I build more? Probably

12au7 starved plate guitar preamp clip 1 https://imgur.com/gallery/dV5ogqh

Sent from my SM-A505FN using Tapatalk

12
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by phatt on September 25, 2020, 04:04:47 AM »
OK I now understand. :tu:
As you have already Found ESP site then dig deep into Rods fine web pages and you will find a lot of gems. :tu:

Be aware that Guitar Amps are so far away from perfect that they bare little resemblance to the Audio Fools magic smoke dealers.

The last thing you want for guitar sound is HiFi flat bandwidth so steer clear blameless amp stuff.
ODrive guitar through a dead flat EQ with 20/25khZ bandwidth is the fastest way to annoy the punters.
Some of the greatest guitar sounds ever heard were produced by what would now be considered a very bad design. :lmao:
As A friend of mine found out when he spent a fortune on a custom build by a hifi teck. He eventually came to me unhappy so I built him an amp from a pile of El crap for 1/10th of what he had wasted and he was blown away by the result.

As for design help if you can find a copy of "Art of Electronics" it's well written for those of us who are less gifted at complex maths. :duh

As noted in the ESP page you quoted, anything running below +/-35VDC is not likely to give much trouble and that covers a basic amp of 40~50Watts so 15 ~20 Watts is fairly safe.
Just wack a few ideas on a bread board and use a Light bulb limiter and see what you can come up with.
I'll try and dig up some old pictures of my testing circuits to give you ideas.
Phil.
13
Tubes and Hybrids / Re: VHT Classic 18 Preamp Modification
« Last post by phatt on September 25, 2020, 03:38:12 AM »


My only questions now are..
How do I properly add a volume pot to the new input?
And is a capacitor before the valve needed?

Also many thanks for your reply's!!! <3)

Hey Tom,
If you want volume control at input then Return to original input BUT insert a 100k ~250k pot in front of C52.
This will open up much better control over how the Amp responds to your rig.

If thee amplifiers overall system tone is bass heavy then you can reduce the value of C52. If on the other hand it lacks bottom end then remove C52.
Also R76 is 100k You can always make it 220k or higher,, that will give the input higher sensitivity.
Don't go crazy as it might squeal like a pig if the input is too sensitive. I'd try 220k.

Also worth note is you have all 3 plate resistors at 220k (being R3, R6 & R30) which to me is a bit too much. I'd be changing 2 back to 100k which might help civilize the outcome.
If you want to get more power stage magic OD then try changing R52 from 2k2 up to 10k or even 20k for some softer power stage compression.
A lot of the magic of the old famous valve circuits was in the power supply voltage drops,, far more useful than buying fancy pants  brand name Valves 8|
Phil.


14
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by shasam on September 23, 2020, 12:43:56 AM »
Thanks!
I have already built little amps, chipamp and class B amp drived by an ampli op (but simplier than this TD 742).
My real goal is to learn how to design a discret one, not just having a new amp.
I know I could buy PCB with TDAxxxx, or used ss guitar amp  for cheap. I could etch Randy Slone PCB too, and follow his schematic.
I just want to learn how to design it myself.  ;)
15
The Newcomer's Forum / Re: Roland JC-120 with weak reverb
« Last post by g1 on September 22, 2020, 12:07:19 PM »
You can't measure DC there as it is blocked by the cap.
The volume control determines the send to the reverb IC, so if you had the volume turned down there will be no AC voltage at the tank.
Check what is getting to the input of the reverb drive IC (IC1).
From what is shown on the schematic, they have 22mVAC signal at the input jack.  Then the volume control is adjusted til there is approx. 30mV at the wiper.  Under those conditions there should be approx. 20mV at pin 10 of IC1, and 1VAC at pin 2.
16
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by Jazz P Bass on September 22, 2020, 11:43:35 AM »
I would build a tried & true amplifier.
Attached is a simple low power amplifier.
Tube Works TD 742.
Build it.
Study it.
Blow it up.  :lmao:
17
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by phatt on September 22, 2020, 08:14:14 AM »
Well an LM1875 can easy produce 15~20Watts.
Just google it, there are tons of circuits out there using that chip, a few to be found here on SSguitar

The LM1875 delivers 20watts into a 4Ω or 8Ω load
from ±25Vsupplies.
Used within specs they are perfectly happy.

As long as using ONLY 8Ω load you can run them from ±30V for even more watts. ~30W would be the limit.

For such a small amp there would be little benefit from designing discrete stuff. Unless of course you just want the experience or learn how to blow stuff up. ;)
I sent many perfectly good transistors to magic smoke heaven learning how to build amps.  :-X
Phil.
18
Amplifier Discussion / Re: Peavey VSS-20 amplifier with chorus noise problem
« Last post by jpcar on September 21, 2020, 07:34:27 PM »
Thanks for the advice!
I found a Xvive reissue on Ebay for $12 + shipping so I went with that. Should arrive in a few days.
I'll post how it turns out.
19
Tubes and Hybrids / Re: VHT Classic 18 Preamp Modification
« Last post by Tomsouthtrader on September 21, 2020, 11:39:17 AM »
Hey,

So there is only 3 IC's on the board, and having followed the traces and double checking the pads.. It does infact just have 3 pins floating and the power pins going to the + and - 15v rails respectively.

But Having now having tried it without the capacitor before the input of the 12AX7 it sounds great!

My only questions now are..
How do I properly add a volume pot to the new input?
And is a capacitor before the valve needed?

Also many thanks for your reply's!!! <3)
20
Tubes and Hybrids / Re: What to do with this old Pye radio?
« Last post by joecool85 on September 21, 2020, 09:32:40 AM »
BTW the volume pot is 1M ohm which is not ideal as its 'off' from 1 to 9 and everything happens from 9.1 to 10. Can I improve that by adding resistors in parallel?

Sent from my SM-A505FN using Tapatalk

This isn't due to the pot being 1M, but rather the taper.  Sounds like you have a linear pot instead of audio taper.  This can be solved with a resistor from wiper (center lug) to ground, between 10% and 20% of pot value.  So 100k to 200k would suffice.
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