FOX: Valve PA head

Details to follow; here are a few photos of a recent acquisition.  A small Australian made PA head, branded as Fox.  It requires repair, purchased as not working - some parts not connected.     

FOX PA AMPLIFIER.  UNKNOWN MODEL

Preamp:  6AU6 pentode (7pin)

Phase invert and power:  2 x 6GW8

Rectifier:   6V4?

Power transformer: A&R Type 1885-80 MA

Output transformer: A&R marked 2676

Controls:  Gram, Tone, Mic

Gram (turntable) screw on inputs on the front

Mic input missing

Multi-pin speaker output on the side

No power on/off switch or fuse.

My favourite feature - the pilot light on a little lampost.  

ACLAM: Mocker

I thought I was about to do my first non-amp-related post in a while, but while typing I realised that this is, of course, based on the fuzz circuit from a Vox UL series amplifier...   It's not an exact copy, but it's a cracking little fuzz regardless of this.  It actually has a really cool gated fuzz tone, it could easily be mistaken for a germanium fuzz.

 

I have previously done a regular 0.1" vero layout for this, but I generally prefer 0.15" board these days. So here it is...

ACLAM MOCKER

I actually prefer running it at 18v, rather than 9v - keeping in mind that the amp ran on 25v.  It's not a dramatic difference, but it is noticeable, especially on the gate control (Bias).   It's not all that fussy about transistors or diodes - keep the transistors low gain, and just some regular silicon diodes will be fine.   Track down some OA200 diodes if you're super keen, but there are probably better ways to spend your time.  Should not be hard to find some BC109 and BC107, if you want some more mojo from the original.

ON THE SCOPE

Just some tweaking of the Bias control here.

Nice to see the actual results being close to my LTspice model.

RCA: Hollywood Star, Model 32263

Apparently I can't help myself, I picked up this highly impractical but cool-looking projector with a matching speaker.  It's an RCA that was made under licence in England.  

Being an English amp, it's running EL84s in the power stage, and quite fortunately, it was full of original Mullard valves - all in great condition, with lots of life left.  

RCA Hollywood Star Projector Amplifier

The signal path is:

EF86 pentode → volume → triode-connected EF86 → James tone controls → ECC83 driver and phase inverter → cathode biased EL84s

The schematic above has been simplified by removing all projector-related components, along with the projector’s mains wiring and switching.  The schematic is also missing a couple of components that appear to have been factory-fitted.  These included a 330pF capacitor from the phase-inverter input grid to ground (pin 7, the boost stage) and a 33kΩ grid-stopper. 

EF86 preamplifier

The first EF86 operates as a high-gain pentode. It has a 100 kΩ anode resistor and a fully bypassed 1 kΩ cathode resistor. Its screen is supplied through 390 kΩ and bypassed to the cathode.

The input impedance is approximately 680 kΩ, set by the grid-leak resistor. A 5 nF coupling capacitor feeds the 1 MΩ volume control, trimming some bass before the following stages.  A 150 pF bright capacitor preserves treble at lower volume settings.

Triode-connected EF86

The second EF86 has its screen connected to its anode, making it operate as a triode.  This gives less gain than pentode operation, and a lower output impedance—roughly 15 kΩ—which makes it well suited to driving the passive tone controls.

It uses a 100 kΩ anode resistor and a bypassed 2.2 kΩ cathode resistor. A 10 nF capacitor couples it to the tone network.

James tone controls

The RCA uses passive James bass and treble controls

The main values are:

• 1 MΩ bass and treble controls

• 1 MΩ and 100 kΩ bass resistors

• 500 pF and 5 nF bass capacitors

• 150 pF and 1.5 nF treble capacitors

These values place the broad centre or pivot region at approximately 320–330 Hz. This is not a sharp cutoff: below this region the bass control becomes increasingly effective, while the treble control has progressively more influence above it.

12AX7 / ECC83 driver and phase inverter

The first half of the ECC83 restores signal lost in the passive tone network. It uses a 220 kΩ anode resistor and an unbypassed 3.3 kΩ cathode resistor. Global negative feedback from the output transformer also returns to this cathode.

The second half is a concertina phase inverter. Equal 100 kΩ anode and cathode loads produce two approximately equal signals of opposite phase for the EL84 output valves.

Each EL84 is driven through a 5 nF coupling capacitor, 10 kΩ grid stopper and 470 kΩ grid-leak resistor. The relatively small coupling capacitors also limit excessive bass entering the output stage.

EL84 output stage

The output stage uses two fixed-bias EL84s in push-pull - cathode biased and bypassed.  DC from the cathode is used to elevate the heaters.

Negative feedback

Feedback is taken from output-transformer secondary pin 4, with pin 3 grounded. It returns to the ECC83 driver cathode through a 100 kΩ resistor.

A 33 pF capacitor in parallel with this resistor mainly affects very high frequencies and was probably included to maintain feedback-loop stability.

Speaker cab

Single 10" alnico speaker, measures ~11DC resistance - everything points to it being a Goodmans.  The cab is cool, but a tad flimsy.   Sounds pretty nice, great mids and highs, which seems common with these.  I had to reglue some plywood that was separating.

Notes and modifications

• B+ is now approximately 320 V, reduced from around 335 V.

• R28, the shared EL84 cathode-bias resistor, was increased from 130 Ω to 180 Ω because the output valves were running well above a reasonable dissipation level.

• The C10A 150 pF bright capacitor across the volume control was removed.

• R9, the first EF86 cathode resistor, was increased from 1 kΩ to 2.7 kΩ. The original stage had excessive gain and was distorting prematurely; the higher resistance reduced the valve current and improved its bias and input headroom.

• R15 and R16 in the bass-control network were replaced. Both had drifted more than 50% above their specified values, shifting the tone-control response too low and substantially reducing the useful range of the bass control.

• R18, a 220 kΩ anode resistor, was replaced after it had drifted to approximately 275 kΩ.

• An additional power-supply filter stage was installed, consisting of a 150 Ω dropping resistor and 33u filter capacitor. This reduced the B+ voltage and provided additional smoothing.

• An undocumented 330pF capacitor from pin 7 of the 12AX7 phase inverter to ground was removed.  The 33 kΩ grid stopper was retained.

• The unused magnetic/optical sound selector switch was repurposed as a negative-feedback on/off switch.

FENDER: Champion 600 reissue

I picked up this Fender Champion 600 reissue recently, and as it had already been modded to some extent and was sounding pretty bad, I thought, why not strip it back and start fresh.

Firstly, is it a Champion 600?  No, not even close.  It's based on a AA764 Champ, with bass and treble controls replaced with a fixed circuit in the amp, and of course it has a silicon rectifier (note schematics below).

FENDER CHAMPION 600 REISSUE

The only useful mod that had been previously done (in my opinion) was changing the grill cloth - apparently the original cloth is notorious for killing high frequencies.  

The stock speaker is interesting - 6" ceramic, 4 ohms. Kind of looks like a little hi-fi speaker, as it has the foam rubber surround.  It doesn't sound terrible, but it certainly can't handle any bass when the amp is running hot.  

Of course, I had to change it...   I picked up a vintage G.B. Equipment alnico speaker, and it sounds pretty darn good.  It's likely to be made by Goodmans by the look of it.  

It has slightly lower impedance, probaly 3.5ohms or thereabouts (it measured about 2.5 DCR), which drops the reflected impedance down a little.  

As part of my rebuild, I removed the low input (cause who needs it), and added a switch to bypass the second triode with a 1u cap (explanation below).  I also added a proper light, as the plastic jewel kind of looks OK, but it's junk compared to the real deal.

Apart from the modded bits, the original board is OK.  Nearly all of the hardware, apart from the power switch and output jack was junk.  Ceramic valve sockets, but at least the 6V6 was attached to the chassis as well as the board.   Transformers seem OK - the output transformer measures at 10k @ 4 ohms, which is a fair bit higher than your standard Champ.  It sounded OK to me, and I may have left it in if it wasn't for the fact that I had a couple of suitable transformers handy.  

Here’s the rebuilt circuit, now converted to a 5F1-style Champ with silicon rectification. I would have preferred to use a 5Y3, but the original power transformer does not have a 5 V heater winding.

I made some adjustments to the layout as I was building - mainly moving things around to make sure parts aren't too hard to access and that the coupling and bypass caps are easy to change. 

 

BUILD NOTES

B+ ~360 V DC

Rectifier / power

UF4007 diodes

22u,  10k / 8u,  22k / 8u

Heater elevated via 6V6 cathode.  2 x 100ohm resistors 

6V6:

Plate:    330 V DC

Screen: 311 V DC

Cathode: 18.7 V DC

Ik​ = 470/18.7 ​= 39.8 mA

$$\text{P ≈ 311.3 × 0.0398 = 12.4 W plate dissipation }$$

$$\text{But that overstates true plate dissipation, because some of the 39.8 mA is screen current.Assuming about 3–5 mA screen current:Plate current ≈ 34.8 – 36.8 mAPlate dissipation ≈ 10.8 – 11.5 WScreen dissipation ≈ 0.9 – 1.5 WSo the 6V6 plate is probably running at roughly:77 – 82\% of a 14 W 6V6 GTA rating or 90 – 96\% of a 12 W older 6V6 GT rating}$$

Output transformer

The original Champion 600 output transformer measured:

10 V AC on the primary

0.2 V AC on the secondary

50:1 turns ratio

10 k reflected primary impedance with a 4 ohm speaker

It has since been replaced with a vintage A&R single-ended transformer rated at 7 k into 3.5 ohms.  

Coupling caps

The original 22nf coupling capacitors were reduced to:

8.2 nF after the first triode

15 nF feeding the 6V6 grid

These changes raise the low-frequency corners from around 7–8 Hz and 28–31 Hz to about 18–19 Hz and 37–42 Hz. These are estimates for the individual networks rather than the complete amplifier response.

The reduced values retain the normal guitar range while limiting unnecessary low frequencies, helping to tighten the sound when the amp is overdriven.

Switchable second-stage bypass

A bit rough around the edges, more gain and a less lows:  A 1uf capacitor can be switched across the second triode’s 1k5 cathode resistor - increasing gain.

Because the 5F1’s global negative-feedback signal is returned to the same cathode node, the capacitor also shunts part of the feedback signal to ground. Negative feedback is reduced as a result of this.

FENDER CHAMPION 600 REISSUE SCHEMATIC

FENDER AA764 CHAMP SCHEMATIC

 

AWA: Model PA1005, 12w Valve PA Amplifier

Another AWA valve PA amplifier from the PA100X series, from the late sixties.  Very utilitarian looking, and very sturdy.  It's a little bit tight in the chassis - not much room to work in there.

AWA PA1005 PA AMPLIFIER

Standard mic, PU (record player) and tone controls.  Bass cut control on the rear, which is part of the negative feedback network.  Designed as a 100v line system, perhaps intended as a portable system.

Slightly unusual in that this has a quad of 6AQ5 on the output stage.  They're described as a miniature 6V6 - they have the same specs at 250v, which is listed as the max voltage that these can handle.  12AX7s used for the preamp & phase inverter.  The valves all tested OK.

The quad of 6AQ5s is cathode biased, the preamp valve heater was used as part of the resistance to ground for the 6AQ5s - some amps do this to provide a DC heater voltage, with the hope of reducing hum. 

B+ is pretty low at around 250v 

Output transformer

Turns ratio of 25.5 to 1 on the output marked as 15 ohm (between the 40 - 60 secondaries).  Despite saying it's suited to 15ohms, the calculations seem to work out better at 8ohms.

A pair of 6AQ5s at 250v wants to see 10kp-p, therefore a quad wants about 5k.  25.5 squared, times 8 = 5,202   

Secondaries:  Common, 40, 60, 120, 200, 315 & 630 ohms

It has quite a chunky mic input transformer: AWA 7XD51768, intended to match low-impedance microphones to high-impedance triode grids.

Components are the same as what's used in many other AWA amps - always good to see mustard caps.

GUITAR AMP CONVERSION

So far, I've done the usual maintenance, safety checks etc.  At the moment, it's a simple two knob volume and tone set-up.  

Work done so far

PU input and associated components removed 

Transformer on the low-impedance microphone input was removed

Input and output jacks added to existing holes

Cathode biasing on the output valves changed to a regular cathode resistor (12AX7 heater in series removed)

First preamp added to the normal heater chain

Decreased voltage droppers from 22k and 47k to 10k, to get more juice to the phase inverter and preamp

Reduced the 220k plate resistors on the preamp - the preamp is closer to a Benson Monarch now.

Replaced the 2m2 grid leak resistor on the phase inverter input with a lower value   

Added a simple tone control

Added a power switch.

6AQ5 Power Amp

150 ohm 10w bypass resistor, 47uf bypass cap

248v on the plates, 15.2v on the cathode 

It’s running pretty conservatively - could drop the cathode bias resistor back a bit, maybe a 135 ohm, but I think I’ll leave it for now.  

FURTHER READING

Tim Robbins - PA1005 #1 

Tim Robbins - PA 1005 #2

AWA: Model PA-806, 5w Valve PA Amplifier

Such an awesome looking little amp from 1954.  Photos from an online listing, I am yet to track one down in person.

For a simple single-ended 6V6 based amp, it's an interesting circuit.

AWA MODEL PA-806 PA AMPLIFIER

Standard inputs and controls for a small Australian PA amp from AWA.  Inputs for a mic, turntable input (PU) with a simple tone control.  No power switch, or fuse.

6AV6 microphone input stage (single triode)

12AX7 mixing / gain stage.  Grid leak biased, negative feedback from OT.

6V6 single-ended output stage, 5 watt

6X4 full-wave valve rectifier

Power Transformer: TS2525823

Power for the rectifier and 6.3v heaters

B+ ~360v

Output Transformer:  1T357100

600, 300 and 150ohm windings on the secondary (plus winding for negative feedback)

This model has also been well documented here

AWA MODEL PA-806 SCHEMATIC