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A valve line pre amp for the DIY fishy

ECC88s ordered cheap from Russia, all other parts I hope to have in stock (need to check for the TX still), and I have a full width case to mount it in (intended to rehouse my Croft Micro II and increase PSU count, but may as well get some experience first) :)

Thanks Jez!
 
Dowser said:
ECC88s ordered cheap from Russia, all other parts I hope to have in stock (need to check for the TX still), and I have a full width case to mount it in (intended to rehouse my Croft Micro II and increase PSU count, but may as well get some experience first) :)

Thanks Jez!
Click to expand...

We have a builder! Beware of Russian ECC88's which aren't ECC88's though. Some are a Russian near equivalent re-marked as ECC88's. It would probably still work well though. It sounds good with Russian ones but I did notice an improvement with Mullard E88CC-01's.
 
Rubbish, how can different makes of the same valve sound different, that's nonsense... Youll be selling Naim upgrades next. ;-)
 
Arkless Electronics said:
We have a builder! Beware of Russian ECC88's which aren't ECC88's though. Some are a Russian near equivalent re-marked as ECC88's. It would probably still work well though. It sounds good with Russian ones but I did notice an improvement with Mullard E88CC-01's.
Click to expand...

Be aware that:

R1, R7 30K

R13, R14 15K

R15 100R

R16, R17 22R

need to be 3W rating and that R1 and R7 (30K) should be metal film or similar. The rest can be wirewound or indeed any type so long as 3W or more rating. All other resistors can be normal 0.6W 1% MF or whatever similar type. The 100R grid resistors should be very close to the grid pins of the valve base, keep the wire between the resistor and valve base no more than about 1/2". Use a star ground.
 
Thanks - ha, no, I know I only need one, but I always buy 2 if I can so I have a semi-matched pair from same batch just in case :)

Richard
 
I would love to have a crack at building this. But I think I need some hand holding having never built or used valve kit.

A BOM would be a fabulous start. I'll try and gather the info from this thread and link it back here. The bit I'm most unsure about is the transformers and PSU.
 
8rNBFjr.png


Here's the BOM (not including transformer or chassis) with RS order codes. Other component suppliers are available! It would seem I've specced 2.2uF on the schematic and 4.7uF on the BOM for the output capacitors. Either will work fine and it's not critical. With the 1N5953 Zener diodes fitted (not on the BOM as I wasn't originally going to fit them) the voltage rating of the output capacitors can be relaxed and anything over 200V working will be fine.

A suitable input selector switch is included in the BOM for those wanting more than one input. The ALPs Blue Velvet vol control is about the most expensive part and it can of course be replaced by any other vol control or switched attenuator.

Please folks, no silly questions about component substitution as the answer will always be the same... anything can be exchanged for a part of the same or better spec from another brand. Do not reduce voltage or power ratings of parts. So yes if you want to use "boutique" brand resistors or whatever then go ahead but if I've said that part needs to be rated for 3W then don't use one of less than 3W... 4W?, 5W? yes fine. Tolerance can be relaxed to 5% or 10% no problem also.

Here's some possibles for a mains transformer:

http://www.ampmaker.com/store/240V-6.3V-toroid-power-transformer.html


http://rover.ebay.com/rover/1/710-5...0001&campid=5338728743&icep_item=281860987264

http://rover.ebay.com/rover/1/710-5...0001&campid=5338728743&icep_item=222212726120

The first one looks the most suitable. If others come across suitable options then we can add them to the list.

Chassis choice is left to the individual but, unless building it with the PSU separate, you will need something large enough to allow some space between the mains transformer and the actual pre amp, in order to keep mains hum down. A toroidal transformer helps here. How much spacing between them? There is no hard and fast rule, the more the better really but as a rough guide allow at least 10" with a conventional transformer or 8" with a toroid. Apart from having a reduced hum field anyway, a toroidal, with its single hole fixing, allows the transformer to be rotated to find the position with the least hum. My "lash up" prototype uses a conventional transformer (TX), with the TX and valve mounted on top of the chassis and other parts below, there is 9" between TX and valve and it's completely silent.

Obviously there needs to be a fuse, mains on/off switch etc which are not shown as presumed obvious. If fitting an IEC power input receptacle (a twat to cut out the hole!) rather than a captive lead then you can get ones with the fuse and on/off switch built in.

Any questions? Just ask;)
 
This site contains affiliate links for which pink fish media may be compensated.
Arkless Electronics said:
8rNBFjr.png


Here's the BOM (not including transformer or chassis) with RS order codes. Other component suppliers are available! It would seem I've specced 2.2uF on the schematic and 4.7uF on the BOM for the output capacitors. Either will work fine and it's not critical. With the 1N5953 Zener diodes fitted (not on the BOM as I wasn't originally going to fit them) the voltage rating of the output capacitors can be relaxed and anything over 200V working will be fine.

A suitable input selector switch is included in the BOM for those wanting more than one input. The ALPs Blue Velvet vol control is about the most expensive part and it can of course be replaced by any other vol control or switched attenuator.

Please folks, no silly questions about component substitution as the answer will always be the same... anything can be exchanged for a part of the same or better spec from another brand. Do not reduce voltage or power ratings of parts. So yes if you want to use "boutique" brand resistors or whatever then go ahead but if I've said that part needs to be rated for 3W then don't use one of less than 3W... 4W?, 5W? yes fine. Tolerance can be relaxed to 5% or 10% no problem also.

Here's some possibles for a mains transformer:

http://www.ampmaker.com/store/240V-6.3V-toroid-power-transformer.html


http://rover.ebay.com/rover/1/710-5...0001&campid=5338728743&icep_item=281860987264

https://www.ebay.co.uk/itm/220V-240V-50W-Transformer-R-type-for-Tube-preamp-265V-0-265V-3-15V-0-3-15V-12-6V/222212726120?_trkparms=aid=222007&algo=SIM.MBE&ao=2&asc=49131&meid=4ab34d127d6d420994d188e542efb720&pid=100005&rk=5&rkt=6&sd=281860987264&_trksid=p2047675.c100005.m1851

The first one looks the most suitable. If others come across suitable options then we can add them to the list.

Chassis choice is left to the individual but, unless building it with the PSU separate, you will need something large enough to allow some space between the mains transformer and the actual pre amp, in order to keep mains hum down. A toroidal transformer helps here. How much spacing between them? There is no hard and fast rule, the more the better really but as a rough guide allow at least 10" with a conventional transformer or 8" with a toroid. Apart from having a reduced hum field anyway, a toroidal, with its single hole fixing, allows the transformer to be rotated to find the position with the least hum. My "lash up" prototype uses a conventional transformer (TX), with the TX and valve mounted on top of the chassis and other parts below, there is 9" between TX and valve and it's completely silent.

Obviously there needs to be a fuse, mains on/off switch etc which are not shown as presumed obvious. If fitting an IEC power input receptacle (a twat to cut out the hole!) rather than a captive lead then you can get ones with the fuse and on/off switch built in.

Any questions? Just ask;)
Click to expand...

Fabulous, thank you very much.
 
This site contains affiliate links for which pink fish media may be compensated.
If anyone is serious about building this I probably have most of the bits needed inc the main transformer and valves.
 
Give me a couple of days and i'll dig out the bits. I know I have a pair of Mullard 88's (although the writing has worn off).
 
Great accessible little project. Thanks very much for posting. Maybe it can become the valve alternative to the Pink Fish/Piglets Dad B4?

I fancy a go at this as a first foray into Valve builds as well. Had a good look at the design and mugged up on some valve theory with Morgan Jones's book.

A couple or three newbie questions if I may please?

The ECC88 is a double triode. I've read it can be used in parallel. Are we doing that in this design or is it one half for each channel? There are two valve bases at the top of the BOM, so perhaps we are leaving one half of each ECC88 unused?

I have a 12v transfomer I can use for the heater but will need to drop the voltage to 6.3v. Will a LM317 based circuit be OK for current at 1.5 Amps max?

Can I make the higher wattage resistors up from 0.5W metal film in series or are wirewound better for other reasons?

Thanks for the inspiration.

John
 
The ECC88 needs about 300-400mA heater current, so a dropper resistor of about 18 Ohms/3W should be about right for 12Vac. Two 33R in parallel would be easier
 
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