ECC807 preamp

[jkeny]

Hi all,
I have successfully transformed the power stage of my salvaged Rogers Cadet III amp into a "Baby Huey" circuit with outstanding results - the sound is so much more - more bass, more detail, more soundstage, etc.

Given such a significant step up in sonics I wondered if the preamp stage would be suitable to similar treatment. It uses 3 ECC807 tubes and includes volume, treble & bass circuits as well as seperate RIAA equalisation (housed in a tube socket called a PU adaptor which I imagine I can reuse as is). I haven't come accross a preamp schematic using these tubes in my searches.

When I knock the case slightly with my finger I hear a ring like a bell in the speakers - is this microphony? I have isolated it to one ECC807 tube position in particular - I don't believe the tube is faulty, just the role that it plays in the circuit as I've swapped tubes and the ringing doesn't follow the tube.

Should I sell these tubes and go a simpler route which could use the Rogers Case or should I use them as is in a better circuit if there is one?

Thanks
JOhn

[richwalters]

Where did you get the ECC807 tubes from ? They are so rare/deare to be near classified as collectors items.

richj

[Shoog]

Benefit can be expected from eliminating the tone stack.
Rare valves are a liability unless you have a few spares.
You need to sketch out the circuit before anyone can make any more useful suggestions.
The RIAA may be difficult to retain if you want to alter the front end, Unless it is a completely self contained unit (which I doubt it will be).

Shoog

[poynton]

Originally posted by jkeny
Hi all,
I have successfully transformed the power stage of my salvaged Rogers Cadet III amp into a "Baby Huey" circuit with outstanding results - the sound is so much more - more bass, more detail, more soundstage, etc.

Given such a significant step up in sonics I wondered if the preamp stage would be suitable to similar treatment. It uses 3 ECC807 tubes and includes volume, treble & bass circuits as well as seperate RIAA equalisation (housed in a tube socket called a PU adaptor which I imagine I can reuse as is). I haven't come accross a preamp schematic using these tubes in my searches.

When I knock the case slightly with my finger I hear a ring like a bell in the speakers - is this microphony? I have isolated it to one ECC807 tube position in particular - I don't believe the tube is faulty, just the role that it plays in the circuit as I've swapped tubes and the ringing doesn't follow the tube.

Should I sell these tubes and go a simpler route which could use the Rogers Case or should I use them as is in a better circuit if there is one?

Thanks
JOhn

Hi.

Yes, that's microphony !!!!!

The ECC807 is VERY rare and sell for a fortune on EBAY !!! It was only used in a very few circuits, your old Rogers being one of them.

You could do a complete rebuild and use ECC83/88/82 .
If not, watch out, the pins are different.

http://www.pamandandy.demon.co.uk/va...0x/ecc80x.html


Andy

[frank754]

As I understand, the effect of microphony is only important in the earliest preamp stage(s), if you can move the tube to a later stage it could perform well for quite some time to come. You can gently tap the tube in question, then swap it out for another and see if that helps.
Use of the same tube in the later phase inverter or driver (etc.) stages shouldn't be affected by a microphonic tube.

[jkeny]

RichJ,
Got the working amp from recycling centre, which I use every opportunuity to repeat as I am delighted with the find.

Shoog,
Yes rare tubes they are - see them on ebay for stg £75 for 3. I have no spares & am unlikely to buy any at that price. Schematic is attached. RIAA is not shown in the schematic but is referred to as PU unit. They made seperate RIAA circuits for ceramic,MM & 78 I think which all come in Plug-in Units (PU Unit). So they are self contained but may have an interaction with circuit.

Poynton, Frank754,
Thanks for the help - microphony confirmed then.

If there is no great advantage to ECC807s I will probably auction them & buy some tubes for a preamp stage to fit in the Rogers chassis.

Thanks for help - please look at schematic and suggest how I could remove tone controls. OOps can't get the schematic below 100k & legible - any suggestions?

In the meantime here's a ECC807 from Brimar that I found. Anybody try this?

John

[gingertube]

John,
I run my Cadet III absolutely standard. The ECC807 based preamp and in particular, the phono circuit is actually fairly good.

I bought an original manual and 3 off spare ECC807 for 75 English Pounds at least 5 years ago when I first acquired the amp. I can therefore keep it original for many years to come. I have not seen ANY ECC807 offered since then.

The ECC807 Mu of 150 is the only reason that Rogers were able to do a Stereo RIAA preamp + a Baxandall (ish) Bass and Treble tone Control with just 3 twin triodes. Changing the tubes to something actually available will probably mean that you have to sacrifice some functionality. One other consideratio, however, is that you could probably get a large heap of cash for the tubes on Ebay.

Why not run it as is for a bit. If you are unhappy with the preamp sound then you can change it later. I run a vintage Pioneer FM Tuner into my unit. I experimented with going into the Tape Input which bypasses most of the preamp and going in the Tuner Input which uses the full preamp but with a different feedback gain and frequency shaping than used for phono and preferred the latter.

The amp drives a pair of vintage Warfdales and is connected to a modern solid state Timer Switch - That is, the system is my bedroom "clock radio" which wakes me up each morning.

Cheers,
Ian

[jkeny]

Thanks Ian,
For the wise words - I will run it stock for a while as I'm still trying minor tweaks to your "baby huey" circuit (which I love) & the Cadet preamp allows me to listen to these tweaks.

I intend to use this as the main vinyl playback system if it all works out, so it will involve more critical listening than a bedroom wakeup alarm system.

This is my first tube based system and I am impressed & excited by it especially as it cost me nothing. So I am keen to see how far I can take it.

John

[Shoog]

I would replace the whole front end with a single triode. All you need to know is what bias voltage your phase splitter needs and work back from that. You may need a two stage preamp to get from an input sensitivity of about 1-2V up to your required driver voltage. The preamp is likely to be compensated to get the best out of the original power stage, but since you have scrapped that you can expect the compensation to be sub-optimal as well.
Sell those lovely ECC807's. Build a better seperate phono preamp.
Accept no compromise !!

Shoog

[SuppersReady]

Hi John,

WRT to the cost implicationsof renovating your Cadet III, I sold 3* ecc807 (NOS) last summer for £89. The Rogers Cadet is generally accepted as being compromised do to price constraints (i.e. built down to a price). I did enjoy listening to mine, but there are definitely a lot of better preamp stages to be built with significantly less expensive valves.

I still have an original owners manual scanned on to a disc somewhere, along with a letter from one of the Roger's engineers which includes the schematics and component values for the various types of PU adaptors. I can email them to you if that's any help.

Paul