The Fisher 125-AX – Restoring Fisher’s 1957 beast!

The Fisher 125-AX, Restoring Fisher’s 1957 beast!

These are so rare I have never seen even one before never mind a pair, and were being sold by “tubetycoon”, whom I had never heard of. Well, this was “The Fisher Doctor”, Al Pugliese in disguise. He’s still the same old larcenist!

The picture shows the mismatched cages, if you look closely, but he never mentioned it in text. The meters didn’t work, and he says they never do, but it is obvious that they have recently been swapped in. Assume the good ones were swapped out!

While he so generously included (readily available) extra meter lamps, NE16 bulbs, and meter covers, one of the two 12AX7 tubes and all eight (!) EL34s were worthless! While Al clearly intended to send bad tubes and parts where indicated, he knows these are transient problems and unlikely to result in a return. He’s done this a LOT. He knows what he will get away with.

Internally, both amps were fairly identical as shown, all original parts. Totally unusable/untestable in this condition. That’s expected though. I am happy that no one (Al!) had attempted to repair or restore before me. This is clearly over his head. One amp had been worked on where the driver section meets the output tubes, and parts remained disconnected. I learned why later, and will explain during the restoration section.

The Plan:

  • Test the transformers as quickly as possible to determine if they are restoration worthy. They were fine.
  • Replace ALL of the capacitors, electrolytic and paper, with modern parts while maintaining cosmetic integrity.
  • Replace resistors where necessary.
  • Replace tubes as necessary to get proper performance.
  • Repair the meters, if possible.

Let’s have a look at the original design!

  • The power supply is a dual-rectifier 5R4GY with a choke-input filter. The 5R4GY is the only consumer grade tube that can handle the PIV that would be in a circuit generating 625 VDC. The massive choke (5.5Hy, 160mA) is the center potted item on the chassis top. This is for real! The following filter capacitors are in series, 2 x 120uF at 450 VDC which nets 60uF.
  • The screen voltage is regulated to an adjustable nominal 285 volts using a 6Y6GT series pass tube, 6AU6 error amp, and an NE16 bulb for reference. The low level audio stages are also powered from this source through additional RC filtering.
  • The bias voltage is regulated by a shunt connected NE16 bulb. This would be a bad idea if the plate voltage varied, but a regulated screen voltage makes this acceptable. Together they will hold plate current constant over a varying line voltage.
  • The low level audio section is Williamson style with a 12AU7 concertina or split load phase inverter, with a 12AX7 driver stage after. An interesting choice by the designer to use the 12AU7 first and 12AX7 after! We can assume this was to put the phase inverter (where a 12AU7 may be a better choice) earlier in the signal path. Note that all cathodes are not bypassed, which lowers gain but increases linearity. Negative feedback is applied at the input tube’s cathode and plate.
  • The 12AX7 driver stage is coupled though high-pass filters (to improve LF stability) to the output stage. This is directly connected to one pair of EL34s, then through 100 ohms to the second pair.
  • The output stage is 4 EL34s in push-pull parallel. The operating plate voltage is 625 VDC, screen voltage 285 VDC, and the plate current adjusted to 45 mA per tube. That’s 28.25 watts per plate, a little over the limit as usual for Fisher!

And the (heroic!) specifications?

  • 90 Watts continuous, 125 Watts peak. The unit will deliver 132 Watts continuously with fresh tubes, but this will exceed the EL34 ratings worse than the 28.25 Watts of idle dissipation!
  • Frequency response within .1dB 20Hz to 20000Hz. Power response at 90 Watts within .5dB 20Hz to 20000Hz. These are Freed output transformers!
  • Distortion below .5% at 90 Watts (they probably only mean harmonic, since distortion type is not specified).
  • Hum and noise better than 92dB below rated output.

I have completed the electronic restoration and have been using the amplifiers a bit since. They are:

  • Loud as hell. There’s still knob left when you can’t stand any more!
  • Amazing bass due to excellent damping & excellent frequency response!

 


 

The Restoration:

Before any testing can be done, these part upgrades need to be done. The original electrolytics were bad by every measure, as these units had probably not been used/played since the early 1960’s. The only things checked out before this were the transformers themselves. A variable transformer was used to apply voltage to the power and output transformer primaries, and all windings are checked for proportional voltage outputs. All OK!

Overview of the upgrades:

  • Replace each 120uF filter capacitor C1, C2 with an F&T 100uF/100uF @ 500V with both sections in parallel, each also having a .1uF polypropylene and a 100K/3Watt resistor in parallel. These assemblies, on the left and right sides in the picture above, are in series. The peak power supply voltage in these units, when unloaded, can go slightly over 1000V!
  • Replace the rear capacitor can C3A/C3B with an F&T 50uF/50uF @ 500V. Section A is the screen supply filter, and section B is the low level stages filter.
  • Replace all small paper capacitors with Vishay/Sprague 715P Polypropylene, rated at 630V.
  • Three pF-range capacitors replaced with silver-mica units: C8, C9, C14.
  • There are several small electrolytics, replace each with 105-degree units of equal value. The exception, C4, was 1uF, can be replaced with 10uF. This improves the decoupling to the first gain stage.
  • Replace the 3.3M resistors at R24 and R29 with matched parts. High value resistors from that era are drift-prone and at this circuit position any drift will make the output tube currents hard to balance.
  • Replace R53 and R54 with matched 82R resistors, as these are where the metering circuit measures the currents on each push-pull side.
  • Check all other resistors and replace if off by more than 10%, regardless of marked tolerance.
  • Replace selenium rectifier M1 with a fast-recovery diode. Because the bias voltage is regulated by the NE-16 Neon lamp following it, no additional series resistance needs to be added.
  • Link to service manual PDF: Fisher-125-AX-Service-Manual

Underside view of the upgrades:

It’s Tube Time:

Tube selection:

  • For EL34 I selected the new Tung-Sol reissues in matched quads, as these continue to gain good reviews and don’t break the bank.
  • For 5R4G, I selected 5R4GBs, as they have a lower voltage drop and are MUCH easier to find in semi-matching groups. They are also straight-sided like the EL34.
  • For 12AX7, genuine NOS Telefunken.
  • For 12AU7, Telefunken was used again.
  • For 6Y6GT, An original Fisher marked tube was used, but the other had developed a tendency to arc-over, so a GE 6Y6GT was used.
  • For 6AU6, the original Fisher marked tubes remain.
  • NE16 bulbs: Any should work. A failure mode that can occur is a drift upwards in the firing (and thus regulated) voltage across these. 53 to 65V is correct.

Fisher had a rather elaborate grading and selection system for these amps to get around a SERIOUS design error. They were dividing the EL34 tubes into groups based on grid leakage, the high-leakage group and low-leakage group. Tubes in two positions were always from the low-leakage group while the other two could be from either group. Fast forward to the day when new output tubes are needed: Without knowledge of this procedure and possession of the right tubes, your Fisher 125 will no longer work correctly! Keep reading…

Now for Testing…

A Modern Driver PCB for the Citation V

NOTE: This is a prototype and I have not fully tested it. Also, The holes are not placed correctly.

When I bought a working, but scrappy looking, Harman Kardon Citation V, I did not dream at the time I would completely tear it apart for a rebuild. I like the design, the output transformers are excellent, so I got the urge to completely rebuild it. While the whole project is ongoing, part of it is to create new, sexier PCBs for the audio circuits and power supply. Here is (re)Vision 1 of the new driver board:

This board is laid out such that each side is a mirror image of the other. All text and designators are printed on both sides. The plan is to put the power resistors on the bottom (which means facing up when amplifier is in operation), and to put the capacitors on the top (which is then down in the amplifier). Being as heat rises, the capacitors will thus be kept cool. Also added is the 10M45 current source in the place of the LTP phase inverter tail resistor. This is also a double-sided layout, and the single part placed will be on the bottom (hot) side of the PCB.

The Altium Designer files are hosted at: HK-Citation-V-Driver-PCB

More to come:

  • A power supply PCB, carrying everything but the doubler caps and screen choke;
  • Tube complement modified to 6AG7, 6SN7, and 807 (overall circuit topology remains the same).
  • Chassis modified for appropriate sockets, chassis and transformer tops stripped and powder coated brown (compatible with original brown transformer cores).

Stay tuned!

Custom Guitar Amp Prototype Build

This project was for a business customer interested in creating a new 55 watt guitar amplifier. The prototype here is for testing the electronic design, and for adding/trying modifications.

The mock layout is shown here, however it was reversed left-right so the inputs will be on the left opposite of the power supply. The goal was to make a smart layout that looked good, with plenty of room for additions or modifications.

Here’s a top view of the chassis with everything mounted. Some items were placed as assembly progressed.

The front view of the same.

Here we can see two things, the power is on with the pilot lamp lit, and the addition of a toggle switch near the output tubes to offer connection options of standard or distributed load.

Assembly proceeding inside.

This is the final product as delivered to the customer for testing. Using 6550 output tubes at 475 plate volts via the 5AR4 rectifier yielded 55 watts pentode connected, 50 watts distributed load. The gain stages and driver tube are all 12AX7s. There was no audible noise or hum with no input connected, so the grounding is good.

More to come as the project progresses!

These Must Be The Best Tubes Ever!


Is this not the absolute coolest tube brand and box? What isotopes do you suppose they use in their proprietary materials? I guess I’ll have to put it in the tube tester to see if the uranium is depleted!

6KV8 Push-Pull Driver Board

Here’s a look at a design project I’m working on. This is a driver circuit for a push-pull amplifier designed around the 6KV8 tube. The 6KV8 tube is a high-gain triode with good linearity and high transconductance coupled with a frame-grid pentode also having a high transconductance. Tubes like this were designed for television applications, not audio. Here the triode sections of the two 6KV8s are connected as an active-loaded gain stage, which then feeds the two pentode sections acting together as a phase inverter and driver section. The pentodes use IXYS programmable current sources to help linearize their operation. The circuit includes local power supply filtering and the negative bias supply. There are two options for feedback loops. One loop goes to the cathode of the lower pentode, and makes a standard feedback loop. The other is plate-to-plate loops from the output stage via connections LF1 and LF2. These are both subject to some experimentation when the prototype PCBs arrive and are stuffed.

This is the art of the PCB itself. This is where my project is different from what you’ll usually see in PCBs for vacuum tube audio. Other than the tube sockets, all components are surface mount. This is the more modern assembly method for electronics. The possible disadvantages are how the PCB will handle waste heat, and the limits on component selection. You can see in red the large copper area used as a heat sink for the current source components, which if effective will eliminate the use of a standard aluminum heat sink.

More to come when the prototype is ready to test!

 

The 89Y SE Headphone Amplifier

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Completed!

In the traditional “Darling” style, it gets .75 watts per channel! The 6C8G is a really nice tube, unfortunately it gets overlooked.

Push-Pull 7C5 Amplifier in Vintage Style

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Here it is, finally! Oh, and I hope you like green! This is a complete rebuild of my very first amplifier. Using everything I’ve learned since that time, I began this project with the goal of creating an artistic and functional version of that design. The tubes, output transformers, and a few internal parts remain from the original build. A new power transformer was used, as well as the new chassis, sockets, and other internal parts. Being mostly the same circuit and parts, it sounds exactly the same.

The schematic shows what few changes were made:

  • The 7C5 screen grids are supplied from the plate B+ through 100 ohm resistors instead of a separate filter section. This is more protective of the 7C5s, whereas the prior connection gave no noticeable improvement.
  • The power supply filter capacitors are reduced to 50 & 50 µF, from 100 & 100 µF. 100 µF is too large a value to follow a standard 5U4 rectifier, and can cause the tube to arc-over internally.
  • The 7C5 cathodes are bypassed to ground instead of to B+, which is known as “Ultrapath”. Other than more expensive higher voltage capacitors, “Ultrapath” resulted in no discernible difference.

The 89Y SE Amplifier – In Progress

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My next project has been cut, painted, and mechanically assembled. This is going to be a small single-ended amplifier with speaker and headphone output similar to the 1626 “Darling” amplifier below. The 89Y output tube is a military grade “type 89 – triple-grid amplifier” with a micanol base and graphite coated glass. Type 89 is a precursor to Type 41, 6K6/G/GT, 7B5, and 6AR5. It has a cap for control-grid connection and separate screen-grid connection pin at the base. Here it will be connected as a triode and driven by the 6C8G. The 6C8G is a tube I haven’t used before which has good linear curves and looks good in the amplifier. It contains two triodes, one of which has a grid connection at the top cap. If both stages prove linear enough and the output impedance isn’t too high, no loop feedback is planned. The schematic I’ve worked up for it is still on paper and not installed underneath the chassis yet. The output transformers aren’t cheapies this time! These are Transcendar 8K:8Ω from Gerry at Transcendar Audio Transformers.

More to come!

Single-Ended 807 Amplifier

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This amplifier was built on-the-cheap. I used salvaged transformers purchased well below online prices and 807 tubes which are still reasonably priced. 7 watts per channel. I won an impromptu local “shootout” with this. Recently added speaker/headphone switch and headphone jack. Now it’s personal!

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How it was done: Single-ended pentode mode, with primary-side feedback, and fixed bias operation.

A look at the construction:

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14N7/14AF7 Loktal Preamplifier

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Here’s a long time workhorse in my setup. Four line-level inputs are on the selector switch, and active/passive mode is selected by the toggle switch in the center. Can easily be rewired to accept 7N7 or 7AF7, so that more types become usable. Collecting tubes is more fun when you can use some!

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The Schematic. This preamp has its two 14N7 or 14AF7 tubes connected as CCS (Constant Current Source) loaded single gain stages. The idea here is to be able to use one stage of gain without feedback. Using the CCS load makes the stage linear enough that it does not require feedback. This does nothing for hum & noise of course, so the power supply is critical and thus somewhat complex. To read more about CCS loading, check out section 11-7 of Vacuum Tube Amplifiers by Valley & Wallman: