Tonefiend Forum

Welcome Guest 

Show/Hide Header

Welcome Guest, posting in this forum requires registration.
Pages: 1 [2]
Author Topic: Webcor TP-4905-1A conversion to Fender Champ?
mwseniff

Posts: 149
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: November 15, 2012, 06:38
Quote

Quote from Digital Larry on November 13, 2012, 23:49
Thanks so much for the detailed response. That first series resistor in the power supply line is 2.2K. I am going to study your remarks and my copy of the "Radiotron Designer's handbook", which my grandpa gave me, compulsively until I can account for every electron. Even the big ones. This is really a case of an old dog trying to learn a new trick. Wish me luck!

P.S. I played the amp for about 10 minutes this morning and it is SO NICE to not have that annoying crackling after it warms up. Completely different experience. Previously I was dreading it.

2.2K makes sense the schematic was hard to read. However keep in mind that the specs may not be right for the tubes currently being manufactured. If the manual has suggested schematics in the back they are worth studying as well.

Digital-
Larry

Posts: 192
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: November 15, 2012, 07:11
Quote

Well, I don't have to worry about current production tubes as I don't think anyone makes 12AB5 any more. They were spec'ed for automotive applications. I've found a fair number of NOS, pretty cheap too, compared to 6V6's.
Digital-
Larry

Posts: 192
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: November 15, 2012, 11:03
Quote

Out of curiosity, when you built your adjustable screen voltage prototype, did you bypass the screen grid to ground with a large cap?

Here is a discussion of adjusting the screen voltage and triode strapping of power stages.

Some excerpted remarks:

Anyway, I've done adjustable screens. Used an IRF820 as an adjustable drop element off-of the B+. Works well (small pot wattage, mosfet doesn't even need heat sinking).

But ... for a guitar amp, I actually like the high screen volts better (i.e., dropped just a bit from the plate by a B+ decoupling resistor and cap). It complements the guitar better. You just have to increase the cathode voltage considerably with a larger cathode resistor.

: When I ran the 807's as a triode (strapping resistor) the amp lost most of the headroom and power.

Most people don't like the sound of triode power stages in guitar amps, seems you just have to have those odd harmonics!
mwseniff

Posts: 149
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: November 17, 2012, 05:35
Quote

Quote from Digital Larry on November 15, 2012, 07:11
Well, I don't have to worry about current production tubes as I don't think anyone makes 12AB5 any more. They were spec'ed for automotive applications. I've found a fair number of NOS, pretty cheap too, compared to 6V6's.

So far, here's my take on how the screen grid operates. Please feel free to correct it.

#1 In a triode, the force pulling the electrons from the cathode to the plate (anode) is proportional to the difference between plate voltage and the cathode voltage (for a given control grid voltage).

#2 When there is a resistor in the plate circuit, the point of which is to create voltage gain from the control grid, this force varies with the amplified signal, because the plate voltage is swinging up and down with the signal. The plate also has a non-trivial internal resistance of some tens of kilohms. This fluctuating plate voltage is the main source of triode non linearity.

#3 The screen is inserted, and kept at a voltage "near" or "sightly below" the plate voltage (with no signal).

#4 As the signal comes into the control grid, controlling the current from the cathode to the plate, the force that the electrons see is now dictated mostly by the screen voltage rather than the plate voltage.

#5 As the screen is not a solid sheet, most of the electrons fly right past it and are pulled into the plate. However some of the electrons smash into the grid [y'know, the BIG ones - wink] and are collected there, accounting for some % of the total cathode current. So, the tube just became a little less efficient because the screen current does not contribute to the plate load current and output signal voltage.

#6 Due to the construction of the screen, it can't dissipate as much power as the plate without melting, and so current limiting is in order.

#7 A fluctuating screen voltage would contribute to either further linearization of the plate current (as in the ultralinear transformer arrangement), or possible further non linearity, depending on its relationship to the input signal. If we're not doing anything like trying to make an ultralinear output stage, then the best bet is to filter the screen voltage using a large cap to ground - otherwise the resistor in the screen circuit will generate a signal voltage and we'll be no better off than before, linearity wise.

#8 The practice of tapping the preamp load supply voltage from the downstream tube's screen supply is probably a cost-saving measure that as a side effect may contribute to the tube amp's "mojo" by either causing overall supply sag (compression) or some signal-frequency variation in the power tube's screen voltage. Otherwise I would tend to take a separate filtered line to power the preamp stage(s).

How am I doing so far?

You definitely want to filter the screen grid supply. In fact most Fender and Marshall push/pull amps have an inductor in series with the screen supply to help regulate it. I prefer well regulated screen voltages personally. As for the triode connection of the screens it is easy to put a switch in to get this mode so why not? It is a sort of an old school sound to it and it's useful in overdubs when recording, it also reduces power output. Stacking a triode track on top of the normal guitar track makes a nice full sound sometimes. I like to keep the preamp supply pretty clean and well regulated for the most part. I even put 1uf @ 450 volt caps on the preamp B+ connection close to the tubes if there is much distance between the preamp and the power supply in the chassis. The extra cap can help increase the punch and generally improves the sound (I figured this out back when I was modding Dynaco PAS3 preamps for the hifi crowd 30 years ago or so).
Digital-
Larry

Posts: 192
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: November 17, 2012, 07:45
Quote

I appreciate your insight and suggestions! So for triode strapping, just connect it directly to the plate then? It won't pull too much current? Thanks again.
mwseniff

Posts: 149
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: November 17, 2012, 13:26
Quote

Quote from Digital Larry on November 17, 2012, 07:45
I appreciate your insight and suggestions! So for triode strapping, just connect it directly to the plate then? It won't pull too much current? Thanks again.

For this amp I would put a 2.2K ohm at 5 watts wire wound or something around there you might not notice much difference if it was a 4.7K ohm 5 watt slightly earlier clipping at most.

Digital-
Larry

Posts: 192
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: November 18, 2012, 07:33
Quote

This guy has written what may be the Guinness World Record article about screen grid circuits!
https://www.oestex.com/tubes/screens.htm

I'm going to see if I can digest it.

Cutting to the chase...

10. GUITAR AMPLIFIERS

Using the above knowledge about the behaviour of Screen-Grids, the following design rules can be applied to guitar amplifiers:

a) BRIGHT, CLEAN SOUND (Minimum Distortion) - eg Lead Guitar, Country, Steel

Screen-Grids to be supplied from a reasonably constant voltage power supply. In simplest form this can be just a large filter cap - say 100 uF after a filter choke (not a dropping resistor) from the Plate supply
Silicon Rectifier - full-wave bridge or voltage doubler
Plates to be supplied from a transient-current capable power supply. Use high-quality industrial grade (high-ripple) large filter caps to plate supply - at least 100 uF per each pair of output tubes (double that for bass guitar amps)
Power Transformer continuous current rating to be double the theoretical output stage Cathode current (to improve regulation)
Screen-Grids preferably supplied from a separate low-voltage (ie 40% of Plate Voltage) supply. Where Plates and Screen-Grids are supplied from a common pwer supply then the filter caps should be as large as will fit into the chassis - preferably 200 uF minimum - the bigger the better!!
Tube heaters preferably supplied from a separate transformer (to prevent voltage drop during peak power output)
Beam power tubes. Tubes must have aligned Screen-Grids (eg 6V6GT, 5881 (USA), 6L6GC/7027, 6550, KT66, KT88)
Output Transformer must have low DC resistance (to prevent voltage drop on peak power signals)
Output Transformer for lead-guitar amps to have low inductance (to rolloff low frequencies and prevent overloading of the loudspeaker - especially in an open-backed cabinet)
ALL interstage coupling capacitors selected to provide -3db rolloff at 50 Hz
No negative feedback loop from loudspeaker
Grid #1 (control Grid) supplied from a full-wave rectifier
Grid #1 stopper resistors to be as low a value as is practicable - eg 100 k Ohms max per tube
Class AB2 or Class B operation

b) SMOOTH, NATURAL SOUND - eg Jazz, Rythm, Folk, Bass

All the above but larger interstage coupling capacitors
Ultra-linear output stage and parallel-push-pull tubes configuration is essential for bass guitar (to reduce output impedance)
Highest practicable loudspeaker impedance - eg series connected multiple loudpeakers (to reduce Output Transformer turns ratio)
Silicon Rectifier - full-wave, full-wave bridge or voltage doubler.
Indirectly heated tube rectifier - eg 5AR4, 5V4G, 5Z4 etc - can be used but good filtering and regulation is needed (Not recommended for bass)
Class AB1 or AB2 operation

c) DISTORTED SOUND - eg Grunge, Heavy Metal, Blues

Directly heated tube rectifier (to ensure high voltage drop on transient peaks) - eg 5AS4, 5U4G/GB, 5Y3GT, 5Z3 etc
Small filter caps - eg 8 or 16 uF (to provide poor regulation)
High DC resistance filter choke (to provide voltage drop on transient peaks)
Plates and Screen-Grids supplied from common power supply
Plates and Screen-Grids at the same DC voltage
Output Transformer to have high DC resistance (to reduce Plate voltage on transient peaks)
Pentode output tubes - eg EL34, EL84
Cathode bias
Class A operation
soggybag

Posts: 84
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: August 9, 2014, 23:26
Quote

I'm obviously late to this party. Thought I'd just chime in anyway. I have this same amp. I got it at a swap meet. It had been modified with a 1/4" jack on the back. Works great for guitar. Doesn't have the most amazing rock sound, it's weak on the treble. But, it's fun to play as a test amp. Here's a link to my blog with a post. https://www.super-freq.com/webcor-amp/
soggybag

Posts: 84
Permalink
Post Re: Webcor TP-4905-1A conversion to Fender Champ?
on: August 10, 2014, 11:01
Quote

I'm obviously late to this party. Thought I'd just chime in anyway. I have this same amp. I got it at a swap meet. It had been modified with a 1/4" jack on the back. Works great for guitar. Doesn't have the most amazing rock sound, it's weak on the treble. But, it's fun to play as a test amp. Here's a link to my blog with a post. https://www.super-freq.com/webcor-amp/
Pages: 1 [2]
Mingle Forum by cartpauj
Version: 1.0.34 ; Page loaded in: 0.216 seconds.

Comments are closed.