Dreams about Monsters

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When is a Dream about a Monster a Good Dream?

...When you're dreaming about building a Monster Vacuum Tube Amplifier.

As mentioned elsewhere, I provide Professional Sound Services for Car Shows and other events. My dream is to build a vacuum tube power amplifier with sufficient power to use as my main live sound amplifier.

This project is in the early planning/design stages, so don't expect much for now. Here are the basic project requirements:

• The amplifier shall produce approximately 100 Watts (TRMS) per channel into 8 ohm speakers. (Almost 200 Watts total output power).
• The amplifier shall be all tube in the analog signal path.
• It will be a straight Power Amplifier. There will be no volume control or other controls/switches, etc., with the exception of a power switch.
• The amplifier shall provide Balanced inputs via XLR and 1/4" TRS connectors. (Separate or combined connectors). It will also have unbalanced RCA inputs.
• As with my other builds, the Monster Amplifier shall have an internal DAC, and provide USB sound card functionality. An Ethernet connection to support one or more industrial A/V over Ethernet protocols is also a possibility.
• As with my other builds, the Amplifier shall use Switch Mode Power Supplies (SMPS) for wide input voltage tolerance, reliability, and weight reduction.
• Sound Quality, frequency response, and Vacuum Tube "sound stage" are definitely goals, but output power is the primary goal.
• As with my other builds, Cost will be minimized by using Chinese sourced SMPS's, DAC's etc., as well as surplus Russian tubes, but Quality discrete components will be used in the analog signal path.

Implementation -- How will the above goals be realized?:

Components:

One of the largest costs of a build like this will be in the Audio Output Transformers. There is a vendor on AliExpress selling 100Watt rated tube output transformers. These transformers have a 3.6K primary impedance which is appropriate for the planned design, and 4 and 8 ohm outputs. The seller seems knowledgeable, and provided specifications and detailed descriptions of the materials used and why their transformers are good quality. Even if they are not on a par with something from a company like Edcor, it is important to keep in mind the goals of this project. While the transformers cost far less than custom parts from a US company, there are significant costs to ship (2) 100W audio transformers from China.

OPT_100W_sm.jpg (34.31 KiB) Viewed 781 times

Also to keep costs low, I will be using surplus used or NOS tubes. Obviously, I'll need 8 Power tubes for a Stereo Parallel Push Pull (PPP) amplifier in addition to driver tubes. There will be 2 stages of amplification between the inputs and the driver tubes, so we are talking about at least (4) Dual Triode tubes.

For power, I am up to my old SMPS tricks. While power budget calculations are still in work, Inexpensive wide input, 12VDC output SMPS's are available at plenty of current.
The 12VDC will directly power things like the DAC board, Panel and/or visual effects lights, and any 12VDC filaments, as well as other SMPS's to provide High Voltage B+, filament voltage, and bias voltage.
The B+ SMPS will be made from a Chinese inverter module rated at 1000W. The square wave output will be rectified and filtered. The module I am looking at has taps for 220V-330V-380V and 420V output (along with lower voltages) and provides connections for feedback for voltage regulation and.

1000W_Inverter_Board.jpg (18.34 KiB) Viewed 776 times

Tube Compliment:

For low cost and at least some sustainability, the amplifier will utilize surplus Soviet military tubes. To achieve the goal of approximately 100 Wrms per channel, the output stage of the amplifier will be of Parallel Push Pull (PPP) design. This means that the amplifier will use (4) 6P3S-E tubes and (2) 6N8S per channel, for a total compliment of 12 tubes. While 6P3S-E are rated at about 250V, and 20W, I have read that they can take much more. My plan is to run them at whatever voltage I end up with from the 330V or 380V configuration of the above mentioned Inverter board (voltage after rectification and Filtering.) bias them conservatively for a Class AB amp, and take the power I get.

In addition to the power stage, there will be an input stage appropriate for either a balanced or unbalance source (I am still researching if an input switch will be needed.), and a driver stage. My initial research/design seems to indicate that 6N8S (6SN7 Equivalent) should be suitable in both input stages. This also has a few other advantages for the intended use of this amplifier:

• Cosmetics -- All "Big Bottle" (Big Base) Tubes
• More secure base/socket than e.g. a 12AT7
• Affordability / Maintenance

The chassis will be built from a surplus 2U rack mount chassis already on hand. As with my other builds, the actual amplifier will be built on the top panel, and the SMPS's and other components will be internal. The chassis will have a "roll cage" or similar to protect the tubes as will as provide a method to make lifting the amplifier more easy.

As with my other builds, there will be some sort of visualization. Right now, I am leaning towards VFD VU meters (as opposed to a VFD spectrum display as in my last build), but real moving coil VU meters, LED VU meters, etc. all remain possibilities at this time. It's also possible that it will contain both DJ - Facing visualization, and Audience facing visualization.

It will also have my "Signature" Lime Green painted output transformers, and Rack Mount handles in the front. In other words, it will follow the design clues of my previous builds -- just much larger.


Status:

It will be well into the next year before I can even make a decision to "Pull the Trigger" on this build.

A schematic is in work, and chassis drawings could start as early as tomorrow, assuming that I can get to the chassis in my crowded shop to take measurements.

I am also working on an informal Bill of Materials/Cost estimate.

More details to follow...

[admin]

"The Monster" Quad 6P3S-E Amplifier Power Budget

As Mentioned, this amplifier will not be based on a conventional 60Hz Transformer power supply.

It will use the "Down-Up" Design that I have been successful with on my previous builds. As explained in the other threads, this means that the AC Lines ("Mains" Voltage") powers only a 12VDC Switch Mode Power Supply. The 12VDC, in turn, Powers separate SMPS's for the High Voltage (B+), Heater Power, and Bias Voltage. To this end, what is below are basic power calculations for each needed voltage. Note that there is multiple levels of conservatism in these calculations. The most obvious is that I round up some of the results. Also max ratings of the tube are used, not average or even the maximum expected loads with Music (signal).

Per Channel Heater Current
4 x 6П3С-Е 4@0.9A each = 3.6A
2 x 6Н8С 2@0.3A each = 0.6A
= 5A per Channel

Per Channel B+ (High Voltage) Demands
4 x 6П3С-Е Plate 4 x 72ma = 288mA
4 x 6П3С-Е Screen 4 x 8ma = 32mA

4 x 6Н8С (ea. element) 4 @ 12mA each =48mA
= 368mA Per Channel

370mA @ 400V = 148W/Channel on B+ Per Channel.

296W total on B+ (Both Channels)


12VDC PS Loads
Heater Bucks
6.3V @ 5A = 31.5W = 2.66A @ 12V Per Channel.
5.33A Total

HV Power Supply
296W/12V = 25 A

Bias Supply (--Deleted. See below.)
Estimate 2A on 12V side

= 32.5A @ 12VDC
= 400W @ 12VDC Power Supply
500W @ 12VDC Power Supply for conservatism.

500W may sound like a lot, but here are a few factors to keep in mind -- Especially when comparing it to a conventional 60Hz transformer and Tube Rectifier power supply:

• 500W 12VDC Switch Mode Power Supplies are readily available.
• Even Name-brand Powers supplies with these ratings can be easily purchased for about $70
• This method provides a regulated voltage, and wide input voltage. Some of the power supplies have a 120/230 input voltage switch, while the better ones will work from 85 to 250 VAC input without needing to be switched.
• The better ones contain an automatically controlled fan that runs when needed to regulate internal temperature. While I would not tolerate a fan in a home Hi-Fi design, A fan is acceptable in an "industrial" amplifier design, and all of my professional (DJ) solid state amplifiers contain cooling fans.
• The power supplies have Power Factor Correction for efficiency, and have AC line filtering in addition to the planned IEC connector with built-in filtering. The amplifier will not put noise back into the AC line.
• Note that the calculations assume 2 separate Buck Converters for the Heaters -- One for each channel.
• The heater Buck Converters provide adjustable current limiting (soft start) and are rated at 200W max, 100W continuous. By using a separate one for each channel, I am limiting their nominal operating power to 31.5W each.
• Note that based on the above calculations, the High Voltage (B+) supply should be able to be provided by the 500W (instead of 1000W) version of the Inverter board -- in other words, the same inverter used in my 7591 Amplifier build. While I need to do a little more homework, the fact that the 500W inverter in the 7591 amplifier does not get hot at all, would tend to support the assumption that the 500W inverter will be adequate, although running closer to it's advertised ratings for this build.

UPDATE: I am considering changing the design so that the +/- 56VDC Power supply is not needed. In my previous (7591) build, I similarly deleted the Bias Boost/Buck Dual Rail converter, and generated negative voltage from an 18VAC winding that is on the HV (B+) Inverter. In that previous build, I used a bridge rectifier to generate sufficient negative voltage to bias the 7591's. For 6П3С-Е's, which have less gain, more (absolute value or negative) bias will be needed, this originally led me to thinking that I could not derive the bias from the 18VAC winding. I then realized that by using a voltage doubler instead of a bridge rectifier, that I should be able to get nearly -36 volts to supply the bias circuit. The additional draw on the HV inverter will be negligible, and no additional active components are used. The generation of the bias will be as fail-safe as any grid bias scheme. The power budget above for "Bias Supply" can be deleted.

Check back periodically, as I have been working on both the schematic, and the layout drawing, and may be ready to share something soon.

[admin]

I have decided to release an Extremely Early Render of this project. Layout and Chassis size are likely to change.

Untitled.png (66.11 KiB) Viewed 331 times

Notes:

• 500W Inverter shown mounted above the chassis, and centered. This is mostly because it was a simple cut and paste from the 7591 render. However the calculations in the previous post seem to indicate that the 500W inverter may be adequate for this amplifier.
• LED VU Meter cut, pasted and stretched from the Poor Man's Go DJ is shown -- again for ease of creating a quick render. This will not be the VU meter used.
• No input select switch/buttons shown.

Edit:
Render Updated. Cosmetic Issues fixed.