DIY Audio Projects Forum

Bruce,

I am finally getting time to build this preamp. I just wanted to verify that schematic in the post below from April 2017 is the most recent, updated schematic.

I recently downloaded and read your article in the Feb 2018 audioxpress magazine regarding the GrooveWatt. Excellent article - thanks again for all that you give to the community.

Regards,

Phil

Very nice build.

I am considering doing PCB's for my build, and have some questions on your build:

1. Which schematic version did you use for your build?
2. Where did you source the chassis with vented lid?

Info appreciated.

Phil

Hi Bruce,

I’m confused about the 220k resistor that in the updated version (April 2017) is connected to the output of the LR8K4 (and are 2 channels!), while in the previous version it was connected to the input (which is common to the 2 PS channels).
Thank you

Bogdan

Hi Bruce and congratulation for your site and for your projects.
I'm Alessandro and live and work in Rome. I'm a digital technician, but now i want to build a tube preamplifier so i'm studing tubes an application.
For preamplifier i have seen your Groovewatt Tube RIAA Phono Preamp Project and i want (i hope) to build it.
So i want to ask you some questions and sorry for my English:

I tried to look in forum the last version of schematic, i read the forum up page 50 and i found the schematic (rev. April 3, 2017), it's the latest version?
You could confirm?

I have a SHURE M95HE MM cartridge and the output voltage signal @1000Hz is 4.7mV and so for the moment I think that will not use SUT.
For your experience it's right?

Refer to the power supply heater section.
You use an adj voltage regulator (LT1085), the circuit in not grounded (ok), the input voltage to the adj voltage regulator is about 10V [(8.5-1.4) x 1.41] and the ouput voltage is 6.25V with 480 ohm and 120 ohm resistors or, 6.15V with 480 ohm and 120 ohm resistor but i don't understand why you have written on pin "E" 68V and on pin "F" 62V?
Could be the voltage measured by voltmeter between pin "E" to signal-gnd and "F" to signal-gnd?

All shields (Power Supply, transformer) are connected to the chassis ground, also the tube shields are connected to the chassis ground?

CH switch for CH 47uF 250V capacitor is a SPDT switch?

What do you think if I insert a timer for the voltage filaments of all tubes?
At power-on only filaments are powered and after 30sec, using two relys 3PDT (one for each channel), all voltages from power supply section are sent on pins "A", "B" and "C". This is a preheat feature in order to preserve the life of tubes. What do you think?

Thanks for all.
Regards.
Alessandro

Hi Alessandro, The step up transformers are optional and not needed for your cartridge. So just skip that section. The voltage measurements on the heater circuit are to the signal ground. The heaters are floating and have a DC reference voltage applied to them through a resistor divider. So the value to ground on one side will be 6 volts different than the other. Typically it will be in the 60-65 volt range. This voltage protects the SRPP stages from tube failure that can occur if the heater to cathode voltage differences get too high. The rating for new tubes is usually 200, but that goes down with age. I figure a more reasonable figure is 150. In a circuit like this it is at least theoretically possible to exceed that if the heater elements in a tube do not warm up uniformly. Plus the reference voltage tends to scavenge stray electrons that don't find their way to the anodes. This decreases the noise in the tube slightly. The 47 uf capacitor is sometimes needed and sometimes not. Also it seems to vary depending on the individual build as to which side of the heater circuit it should go to. So since it is easier to build in the capability than to add it later I include it. There is no switch though. I use the a pin style header with three pins. Center is the capacitor and the sides go to the respective sides of the heater circuit. Then you can test it after you get the preamp operational by placing one of the small two pin jumpers often found in computers (and other places) on each set of pins in turn to see if it matters and if so which way is better.

The tube shields do need to be grounded. Since I have the tubes inside the chassis (highly recommended for hum and noise reasons) I find they work best grounded to the signal side and not the chassis. External ones need to be grounded to the chassis to comply with electrical code requirements.

I find there is no advantage to having a B+ delay in this circuit. Tube life is measured in decades. None of the ones I have built (now 8 years ago on the first one) have ever had a tube that needed replacement. Since they are used rather conservatively in this design a tube life of about 10,000 hours is likely. Some of mine have close to half that now. The only reason to replace a tube in this preamp would be because it was either defective or started to get noisy. None of mine have. In the large power amps I do use a delay. In them the power supply can deliver huge amounts of current at very high voltages immediately. It may extend the life of the tubes in that situation. The jury is out on that and there is considerable controversy if there really is a problem. With the high power amps it also keeps the actual operational B+ below the ratings of the filter capacitors I use (500V). Without the delay it would go up to about 550 until the tubes conducted it down.

The April schematic is the latest.

EDIT: If you can find a copy (print or electronic) of Feb 2018 Audioxpress magazine the preamp is written up there.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Bruce I appreciate your answers and advice.
I will try to look or buy Audioxpress magazine of Feb 2018 and then I will start to build phono preamp.
First I'll have to check the parameter WAF. WAF is the acronym of Wife Acceptance Factor .

Regards:
Alessandro

Use a wood outer case....

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Bruce,

Thanks again for all your help and advice. I am thoroughly enjoying my Forewatt build, and looking forward to a Groovewatt to add to the system.

I've come up with a PCB layout for the tube section of the preamp, as I've been couch-bound recovering from a procedure, and occupying my time learning Kicad. I've borrowed heavily from other builds in this thread. I've attached my first cut at the board, and was wondering if you could point out any obvious issues that should be addressed. The board routes the signal paths on the bottom layer (green) and power (B+ and heater) and ground on the top layer.

The build will use the Audyn True Copper caps, and is based on the most recent schematic referenced in the AudioXpress article.

I appreciate any input.

Phil

Hi Phil, Nice work. It is difficult to be sure , but it looks OK. I don't see any trace width indications. Use as wide as you can and as thick as well. All our commercial boards use 3 ounce copper traces for most connections. Watch out for signal and power traces that run parallel to each other. It seems there are a lot of long parallel traces. Much better to be less "pretty" and much shorter. Length and parallel traces can introduce noise. * We don't have a PCB for this preamp yet, but the one in the works. It uses lots of ground planes and the power and signal ones are brought out separately to a main external ground. I can't tell on yours if you do this or not. As most everyone knows I really hate hum and noise and I do anything that will reduce it. I would not be surprised that you will need to make changes after the first preamp is built. ( I would not buy more than one pcb if you get them commercially) We have been through at least 8 prototypes. The build in the Audio Express article has the latest features and layout. The traces on the most recent pcb go in mostly the same locations. Sorry but I can't make that board design available.

* If you must have some parallel...you can put a ground trace between them. Use the signal ground for that application. It need not be really thick.

Good listening
Bruce

_________________
Some of my DIY Tube Amplifier Projects:

• PoddWatt: 5751 / EL84 Push-Pull DIY Tube Amplifier Project
• OddBlocks: KT88 Push-Pull DIY Tube Amplifiers (Mono Blocks)
• ForeWatt: ECC802S (12AU7 / ECC82) DIY Tube Preamplifier Project
• GrooveWatt: Tube (Valve) RIAA Phono Preamplifier Project

Thanks for the comments, Bruce.

The fill on the top layer (where all the power runs) is signal ground for the board. Is that a good/bad idea?

I can add separate board connectors for the B+ to shorten those lengths. The heater runs are long parallel runs - are there better options?

Very new to this level of complexity...

Thanks again.