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11

Tube Amp Building - Tweaks - Repairs / Re: Negative Feedback resistor value for bass

« Last post by pbman1953 on April 24, 2026, 08:40:37 am »

You're probably right, someone suggested a 1.5k. Any thoughts on that? Thanks!

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Tube Amp Building - Tweaks - Repairs / Re: Negative Feedback resistor value for bass

« Last post by sluckey on April 24, 2026, 08:25:27 am »

Fender used the 820Ω/100Ω NFB resistors in a lot of the blackface amps, including many blackface Bassman amps. Don't get confused by the feedback resistor values in other amps that use a different power amp circuit. Maybe replace the 820Ω with a 470Ω in series with a 5K pot. Then experiment. I suspect you will spend a lot of wasted time and eventually go back to the 820Ω.

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Tube Amp Building - Tweaks - Repairs / Negative Feedback resistor value for bass

« Last post by pbman1953 on April 24, 2026, 07:57:49 am »

Another question on the Super Twin. I see that the stock NFB resistor is 820 ohms but that was for a guitar amp. I use it as a bass head. Plus,  I see for the SVT they used a 47k and Traynor used a 100k.


What would al think would be the right values to test out?

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Tube Amp Building - Tweaks - Repairs / Re: AO-35 Schematics

« Last post by sluckey on April 24, 2026, 07:16:59 am »

I doubt you will directly find such a list. Google is your friend. Spend a day surfing google and you can put together an extensive list. Most of the conversions will be based on the Vox AC-15 and Marshall 1974 18 watt.

Lots of Hammond conversions (including AO-35) here...

     https://ampgarage.com/forum/viewtopic.php?f=6&t=24331

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Tube Amp Building - Tweaks - Repairs / Re: Zener Bias "Trick" for Cathode Biased Amps

« Last post by HotBluePlates on April 24, 2026, 05:48:35 am »

... 1/4 Za-a ... for class B, and ... 1/2 Za-a ... for class A?  ...
https://www.valvewizard.co.uk/pp.html

If there is current pulled through a half-winding then it is "present in the circuit."

   - Class A has current pulled at all times, so both half-windings are "present in the circuit."
   - We get the rated Za-a, and each side sees 1/2 of that Za-a between itself and the power supply.


   - "Class B" (or Class AB once one side has shut off) has no-current through a half-winding part of the time.
   - When no-current flows, that half-winding "doesn't exist."
   - The remaining half-winding has 1/2 turns, and (1/2 turns)² = 1/4 Impedance


We could do a math proof of this using a common output transformer, and recalling that if current flows and there is 1-volt-per-turn on any winding, there is 1-volt-per-turn on every winding of the transformer.

     8kΩ push-pull to 8Ω = 1000 to 1 Impedance Ratio
     √(1000 : 1) = 31.623 : 1 Turns Ratio

     Assume 20w across 4Ω primary.  Volts on Primary and Secondary are:
     Primary:  Volts = √(20w x 8kΩ) = 400v RMS
     Secondary:  Volts = √(20w x 8Ω) = 12.65v RMS
     Volts Ratio = Turns Ratio = 400v / 12.65v = 31.62 : 1

     Both Sides Conducting:  Each side of the Primary sees a load of 4kΩ
     
Now assume there are half the turns on the Primary, because one side is shut off.  The Turns Ratio must also be halved:

     Turns Ratio = 31.623 / 2 = 15.812 : 1

     20w into 8Ω is still Volts = √(20w x 8Ω) = 12.65v RMS
     Primary Volts = Secondary Volts x Turns Ratio = 12.65v RMS x 15.812 = 200v RMS

     Current = Power / Volts, and Impedance = Volts / Current, manipulating gives you Impedance = Volts² / Power
     Impedance = 200v² / 20w = 40,000 / 20w = 2000Ω

     ---> 2000Ω is 1/4 of the original 8000Ω.  We wind up seeing that "Impedance Ratio" fell from 1000:1 to 250:1 when the number of turns was halved.  We could also note that when we calculate Power using known-quantities of Volts, Current and Impedance that the term for either Volts or Current gets squared... and that's where the "squaring" comes from for "Impedance Ratio is the square of the Turns Ratio."

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Tube Amp Building - Tweaks - Repairs / AO-35 Schematics

« Last post by Texson on April 24, 2026, 12:36:50 am »

Does anyone know where I can find a list of schematics for the various ao-35 conversions? I'm not sure which conversion I want but would like to check out a few schems while I am deciding.

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Tube Amp Building - Tweaks - Repairs / Re: FILMOSOUND REBUILT

« Last post by Graydigger on April 23, 2026, 04:54:42 pm »

The old Filmosounds certainly fun projects, and the chassis’ with their compactness, tube socket complement, and old iron lend themselves to lots of design imagination!  Plus 80+ year-old vintage stuff is just plain cool.

A recurring theme does seem to be a propensity for noise and hum, along with where to stuff everything when doing a conversion.  I thought I’d post a video which is more intended as a “Filmosound PSA” for those who don’t have a lot of experience working with these chassis’, but are contemplating wither a restoration or gut and convert… hopefully it has something that’s useful to someone!

i=9rs4ysMw6TggDQeM[/youtube]

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Other Topics / Re: Meme of the day

« Last post by shooter on April 23, 2026, 09:39:21 am »

...

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Tube Amp Building - Tweaks - Repairs / Re: A Question about system design

« Last post by AlNewman on April 22, 2026, 06:50:45 pm »

Perhaps more versatility...
And it's always fun to forget your tone knobs are down and think your amp is broken.

20

Tube Amp Building - Tweaks - Repairs / Re: Layout once-over?

« Last post by acheld on April 22, 2026, 03:41:08 pm »

Vactrols work fine, though my personal preference are the low voltage Omron relays.   If you do use the VTL5C1's, XVive is a really good imitation of the original Perkins-Elmer units from ages past -- there are other cheap knockoffs that are not reliable in my personal experience.

There are versions of this circuit which use relays.

The problem with Slo builds is that layout really matters.  The high gain nature of the beast makes it so, even a low power version.  A small chassis might give you trouble. Or maybe not, but it did for me in my few high gain adventures.  Only one way to find out.  Just build it.

Use shielded cable in the signal path until you can't . . .

I'm having trouble reading your schematic because of the yellow.