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Originally Posted: Sat Oct 15 2022 15:46:48 GMT-0400 (Eastern Daylight Time)


Here's a Minor Update, Cross Posted from AudioKarma.org, with edits.
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Engineering is always a compromise.

I don't like compromise.

...So I compromised.

Many articles debate the value for the Grid Bias Resistors for 7591 based amplifiers -- Particularly Fishers, but this applies to other grid biased 7591 amps, too.  Many of these articles suggest installing 220K, or even 200K, with the lower values helping to prevent bias run away. So I had previously installed 200K Grid resistors.

However, I felt that the Amp (only a single channel at this time), was not putting out the power that P-P 7591's should produce -- probably because the Phase Inverter was being loaded down by the 200K Grid Bias Resistors.

I didn't think that the change from 200K to 220K would make a lot of difference on the load on the PI; But going to 270K worried me, especially with my experience with (used) 7591 family tubes.

240K is also a value for Metal Film Resistors, and an assortment pack I bought from MPJA contained this value -- Yes, that's an unsolicited plug. 2W is overkill for a grid resistor, but 2W resistors in the values included in that assortment have lots of applications in Tube Amps.

So based on totally empirical "guess the number", I installed the 240K resistors.

The amp is definitely making more horsepower with theses resistors installed.  For all you Fisher owners, and other Owners of 7591 based amps that may be worried about run-away tubes, but don't want to hurt your sound, 240K may be the value for you.

I also changed the diodes on the Bias Board to High Speed Glass Diodes that are more appropriate for the High Speed Switching PS that is providing the 18VAC for the (-) bias.

Originally Posted: Sun Oct 16 2022 15:10:36 GMT-0400 (Eastern Daylight Time)


Just another quick update:

The other Channel is complete, and playing music.  I have installed the brand new 7591a's in the left channel, and set the bias.  The amplifier is playing in stereo.

Supply voltages all look good (391VDC on the plate, and 349VDC for the screen/pre-amp/phase inverter supply).

During build up of the 2nd Channel, I noticed that I used a 3.9K resistor on the cathode of the Channel 1 pre-amp.  I had 1.8K on the schematic, with 2.7K written in as "as-built".  I don't have any other notes or memory of where I came up with the 3.9K value, so I built Channel 2 with 2.7K.  I like the voltages around the Preamp/PI better on Channel 2, so I will change Channel 1 to 2.7K.

With the built-in Spectrum Analyzer disconnected, the amp is dead quiet in idle.

With both channels working, I now have a full load on all power supplies.  Nothing but he power tubes are getting hot.

I am still suffering from Dummy load anxiety, and I don't have a pair of dummy loads appropriate for continued feedback tuning.

My intent is to complete final assembly (Chassis sides, handles, etc.), and in the mean time maybe-- order some dummy loads.

Pictures soon.

Originally Posted: Sun Oct 16 2022 22:23:31 GMT-0400 (Eastern Daylight Time)


As Promised, a Couple of Pictures:







Notes:

• No High Voltage Cage.  Home Depot was out of the perforated metal sheeting I was going to use.
• There is still protective masking tape on the top.
• Front Lens/Bezel not installed -- Again, this is to protect it from scratches, as I continue building.
• Only the 2 Tubes on the left are the new ones purchased for this project.
• Not Visible, but the little metal brackets that allow the top to remain stable when the unit is opened are still installed.  There is still a non-trivial amount of work to be done inside.
• Little Rubber Feet (LRF) installed.  That's kinda like when an airplane under construction has "Weight on Wheels" for the first time.

UPDATE:



Added picture of back panel.

• Note USB/SPDIF Optical/SPDIF Coax inputs on the left.
• DAC Volume Knob installed.  Note that this is a digital encoder, there are no wipers, and no audio flows through this device.
• Only the 8 ohm terminals are currently connected internally.
• Hole plugs installed.  At this point, it is hard to tell that this chassis was re-used from another device.
• Hinge brackets to allow easy servicing are visible in this view.  These will be removed.

Originally Posted: Fri Oct 21 2022 21:45:29 GMT-0400 (Eastern Daylight Time)


This project is (kind of) on hold again.

There's nothing wrong with the project, it's just that I don't have 2 decent audio dummy loads.

I had a whole separate project in mind to make an "Audio Tester" that would include dummy loads, but I have scrapped that idea for reasons that I will not get into here.

So why is it "kind of" on hold?  While the Amp itself has been off the bench for a few days, I have been working on the cover for the High Voltage SMPS (The board seen on the top of the chassis in other pictures.)

The original plan was to bend it from a sheet of perforated metal; However, I decided to make it out of materials I already had on hand.  Namely Lexan (Clear Polycarbonate) and steel mesh.




It may get one more coat of Polyurethane, but it is basically complete.  I was thinking of providing some lime green trim (Opposite scheme as the OPT's), but I may just use some green LED's mounted inside to achieve a similar, but more intense effect.

Maybe a "Warning High Voltage" sticker?

Originally Posted: Mon Dec 26 2022 15:40:58 GMT-0500 (Eastern Standard Time)


For now, I am considering this project essentially complete.

Here are the official "Glamor Shots" of this project:





It generally sounds great and has plenty of horsepower and bass. It also has the clear, crisp highs one would expect from a tube amp.

I still may do some more work on the Driver/PI circuit in the future if I learn more.

I also may still do some chassis modifications, including:
   

•  2 holes just under the Spectrum display for access to the clock/mode set buttons. Right now you basically can't set the clock, but I don't think the clock display is important.
•  I may install a toggle switch to the right of the display to allow the Spectrum analyzer to be turned off.
•  I need to install some reflective tape inside the HV enclosure to keep the Green LED light from leaking through the paint at the front.

Originally Posted: Mon May 29 2023 09:57:36 GMT-0400 (Eastern Daylight Time)


In my last post, I stated that this project was "essentially" complete.  Now I will demonstrate why I didn't just say "compete".

After some use and testing, I decided that I was not happy with the VFD (Vacuum Fluorescent Display) Spectrum analyzer that I had purchase and installed in this amplifier.  Here are some specific complaints I had about the VFD SA:

• It put significant noise back onto the audio lines.
• It defaulted to a Clock mode. I was not trying to make an overpriced clock radio.
• Testing showed that the spectrum analyzer was highly inaccurate, and did not respond at all over significant parts of the audio spectrum.
• The display was "OK", but again, for what is supposed be a "high-end" piece of audio equipment, I wanted better.

After some research of available bare VFD SA boards available on AliExpress, I found out the board I had bought was essentially "counterfeit", or a poor quality version of a board known as "AK2515".

On big hint is that the knock-off, or poor quality VFD's do not even specify a frequency range in the AliExpress advertising.

But even "real" AK2515 boards still have the clock mode.

A little more research lead me to this unit from Douk Audio:



This unit does have a full set of advertised specifications.  It does not have a clock mode, and while there is no question that Douk Audio is still a "Chi-Fi" company, I at least trust them to have a little better quality than most of the other Chinese vendors.

The board still needed some connector modifications, and I had to drill a new hole in one of the support brackets, I was able to install the Douk Audio VFD into the amplifier without too much difficulty.

The new display is slightly larger, and noticeably brighter and "bolder" than the previous one.

I will point out that I still consider all of these to be highly inaccurate for actually measuring audio frequency -- especially in stereo display mode.

First, there are 25 bars, and there is no "blank spot" or skipped bar in stereo mode.  This means that the left channel has 12 bands and the right channel has 13 bands.
Furthermore, these displays don't actually have an analog filter for each band.  They use a microprocessor to do DSP and scan the display.  The result of all of the above is that even if the same pure sine wave signal is fed into both channels, the left and right channels will not display the same levels at the same band anyplace in the audio spectrum.

That said they still provide a visually interesting visualization of the music, and they have to the bright, and attractive color that only VFD's can provide.

I've posted a short clip of this amplifier operating. See YouTube Description for details and track playing.

https://youtu.be/j3-kpuOCmQc


The track is a 48KB/s FLAC file unchanged from the file I purchased via BandCamp.

Note that Input 1 (First set of RCA Jacks) is selected (Left most Blue LED is illuminated). I honestly don't remember the exact audio source/path for this video, or why it is analog. I normally listen with via my Nvidia Shield TV box, which is connected to my TCL TV via HDMI, and the TV is connected to the amplifier via S/PDIF optical -- Thus using the on-board Hi-Fi DAC.

I will also say that this amplifier is rarely used these days. My TCL TV does not adjust the S/PDIF volume (But the Shield TV does adjust the HDMI Volume before it gets to the TV). I also rarely watch TV, and not everyone in my house wants to hear me cranking this amp and my Cerwin Vegas.

While I can't use this Amp as evidence that my use of SMPS's to power tube amps withstand the test if time, I can report that my Subminiature Tube amplifier has been in near daily use for almost 8 years. The external 12VDC power brick failed about 6 years ago and was replaced. I have not done any other changes or repairs to that amplifier. My 6BQ5 amplifier is now my shop/bench amplifier, and sometimes runs all day in an 80+ deg F environment. It has been trouble-free for almost 5 years.