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27 tube preamp using Bartola gyrator

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I am intrigued by Ale Moglia's schematic for a 27 tube preamp using his gyrator as a hybrid mu follower. (Schematic attached.) (Note that the 27 tube is IHT, NOT DHT.) I do have a few questions I hope someone can help me with.

First, the heater requires 2.5 volts and 1.75 amps. I have read that ac is fine, would regulated dc be better? Could I use 5 volts and put both heaters in series? I assume that would require a 5 volt transformer with 1.75 times 1.8 amp capability (i.e., 3.15 amps, could I get away with 3?)? Or, if dc, a 6.3 volt winding that I regulate down to 5 volts. Would 2.5 volt in parallel be better?

Second, Jose Martins in an entry on Ale's blog says that you have to tie the cathode to the heater ("In these old triodes it is crucial to connect the cathode directly to filament according to the instructions of data sheet. Without this connection the hum noise is very loud.") http://www.bartola.co.uk/valves/2016/08/25/2756-preamp-from-jose-martins/ I do note that the 56 tube datasheet (56 is a replacement for the 27 tube) states "The cathode should preferably be connected directly to a mid-tap on the heater winding or to a center-tapped resistor across the heater winding. If this practice is not followed, the potential difference should be kept as low as possible." https://frank.pocnet.net/sheets/021/5/56.pdf Does this make sense? Wouldn't the heater power supply pollute the signal? If that is not a problem, what size resistor should be used? If you do this, do you not have to worry about lifting the heater voltage? On the other hand, if you lift the heater voltage do you not have to do this?

Thanks for any help anyone can give me on this.
Jazzzman
 

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I made a 37 linstage very similar to this but with a CCS in the anode. That was a few years ago before I learned about the gyrator. I've always thought it sounds very good. 37 is a 6.3v heater version of 27. I elevated the heater CT with a cap bypassed voltage divider off B+ and haven't had any hum issues. I used cathode bias with a simple resistor. Mine is based on a design Gary Pimm had on his old web site.
John
 
Thanks, John. Is your heater voltage ac or dc? (or should that be obvious, I suppose you might run into problems if you connected your ac heater voltage to the B+?) If dc, did you regulate it?
Stocktrader, I am not sure I understand your question regarding output level.
 
A year or two ago I tried a very similar circuit with a 27 as the second stage in my phono preamp. I used a plate CCS and set the bias with an unbypassed cathode rather than battery bias. Pretty close to the same operating points shown in the OP's schematic.

I started with 2.5v AC on the heaters and both 27 tubes in parallel. There was a very small amount of hum but not enough to be a bother. I listened to this for a few weeks and then switched to a 2.5v DC supply---nothing fancy, just a simple FWB with CRC filtering. To my surprise the DC supply sounded better than the AC and as a bonus the hum was completely gone. I was surprised because each time before this that I have tried AC vs. DC on heaters, I preferred AC. I don't know why it was different this time.

I also tried a couple 56 tubes in place of the 27, and I preferred the 27 each time. A little less gain but subjectively a bit more detailed and deeper bass. I wouldn't call this anything conclusive since I only had two pairs of 56 tubes (National Union and GE) whereas I have many 27s to choose from. My top choice for each was National Union for what that's worth.

I eventually scrapped the 27 stage and went back to my old standby, a 5687 with plate CCS. This was also a surprise. As much as I like the sound of the 27, in this particular circuit the 5687 sounds better at least in my system.
 
Thank you, Salectric. Your experience is quite helpful. You wouldn't happen to have a schematic of your power supply, would you? If so, would you mind posting it?
I am leaning toward the 27, but may try the 37 instead (given the more customary heater voltage). Your impression of the 5687 also seems interesting, but that tube appears to have much more gain than I need.
Thanks again.
 
I think the matter is coming into better focus now, thank you. It looks like a dc heater might be advantageous, so I am thinking of finding a 6.3 secondary, rectifying, filtering, and regulating the ac voltage down to 5 volts dc and then wiring the heaters of the two tubes in series. I think that 3 amps should be good enough for 5 volts in series. I then can either lift the heater voltage or make the connection between cathode and heater mentioned in the data sheet.
Now I just need to hunt for the appropriate transformer(s)!
Thanks again.
Jazzzman
 
If you run the pair of 27 tubes in series, you should ground the junction point of the two tube heaters (and do not otherwise ground the 5v supply).

Personally I would not do the series arrangement. There is such a variety of 27 tubes that the heater windings may differ slightly thereby skewing your 5v supply to favor one tube over another
 
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