• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Possible tweak

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Have SE 2A3/45 mono´s with a regular Interstage.

Lot of attention in the power supply... the sound very good but can i better it

--One possible advantage is that you can now `autotransform` the IT.
This exact application is what Langeford-Smith discusses in his section on parafeed.
Pity the MQ transformers aren't wound this way, but you can easily do this by connecting the primary - to secondary +.

Can i place a capacitor ....between primary - and secondary + ....?
Bypassing the IT.

Just optimize and use both in one SE amp...
I try to `squeeze` the best out of the IT and the best out of a V-cap or Amphenol cap i have ...

Langeford-Smith anywhere to be downloaded...
 
Last edited:
Administrator
Joined 2004
Paid Member
If your IT is 1:1 and phased correctly you can place a capacitor between the plate end of the primary and the grid end of the secondary.. This in theory should almost eliminate the effect of leakage inductance between the primary and secondary. In practice you will have to try it to see whether it is an actual improvement.

Your auto-transformer idea might also work, just remember to make sure that you maintain a dc path for the output tube grid. (resistor) I would expect this to provide additional gain if phased correctly, but at the expense of HF bandwidth.
 
Thank you Kevin ...the IT is 1:1.
to picture it....
Primary side.. plate side of the driver tube (2a3) B++ 250v

Have used (call it sec. + ) a grid choke for the 45-2A3 -output tube
And fixed bias as well -45v (to mention as sec. - )

Can i try it...?

Later i replace the gridchoke and have a 1K there to mention differences ...
With good resistor to the grid of the output tube might work as well in combination with the V-cap
over the IT.

Because these SE mono´s are sensitive -include to change- can hear the differences and nuances clearly.


I quote --One possible advantage is that you can now `autotransform` the IT......a.s.o.
 
Last edited:
Administrator
Joined 2004
Paid Member
Thank you Kevin ...the IT is 1:1.
to picture it....
Primary side.. plate side of the driver tube (2a3) B++ 250v

Have used (call it sec. + ) a grid choke for the 45-2A3 -output tube
And fixed bias as well -45v (to mention as sec. - )

Can i try it...?

Later i replace the gridchoke and have a 1K there to mention differences ...
With good resistor to the grid of the output tube might work as well in combination with the V-cap
over the IT.

Because these SE mono´s are sensitive -include to change- can hear the differences and nuances clearly.


I quote --One possible advantage is that you can now `autotransform` the IT......a.s.o.


Can you post a schematic? I'm a bit confused by your reference to grid chokes and 1K resistors, normally a choke would not be used with an IT.

Using the IT as an autotransformer would increase the source impedance the grid of your DHT sees by a factor of 4, and increase the gain by 6dB. The increased source impedance may result in an early HF roll-off depending on the quality of the IT and the rp of the driver.. What is the driver tube - 2A3? If this is the case i can understand your desire for a step up as the gain must be rather low, give the low rp the transformer probably determines the HF bandwidth due to leakage inductance.
 
Kevin,
Yes the driver is a 2A3..

...tried following..
Used a IT- gridchoke - 45~2a3
Used IT- resister 1k - gridchoke - 45~2a3
Used IT - resistor 1k - 45~2a3

Thought about even using the gridchoke parralled with a resistor ..or resistor parralled with the little choke- two in one in stead of two after each one but do not know if wise or of any use.

Have a lot to gain or loose at this point in the amp i feel ...

Try out one:
IT - resistor 1k - 45~2a3
The cap over the IT ...is it allright ..?

In theory software could measure and stimulate first but cannot do that.
 
Last edited:
Tube Circuit Gain Calculator tells me the 2A3 gives a
GAIN: 1.93 now.

Plate resistance 800 R
Plate load resistor 3500R
following grid resistor 1000 R
Transconductance 5250 R
Cathode resistor 45~2a3 at 10 R

Grid resistor at 2k gives GAIN :2.45
Grid resistor at 5K gives gain of 2.87

So what is the acceptable gain needed for the IT then being a 3.5k 1:1......?
 
Last edited:
exeample EMl20B

EML 20B is being advertised as the best driver in town.

Plate resistance : 9767 420v/0.043amp.
Plate load resistor: 3500
Following grid resistance :1000
Transconductance: 5900
Cathode resistor: 10

GAIN :4.1

Difference with the 2A3 at f.e. 2K .... 2.45 should work or ....better up the resistor to f.e. 6k with a cathode resistor of 5 R have a gain of 3.01 ...
 
Last edited:
Administrator
Joined 2004
Paid Member
Is the 1K resistor a grid stopper in series with your 2A3 output tube grid or in parallel with the IT?

If in parallel with the IT you should replace with no less than 5K or remove it entirely.

You will get a gain of ~3.8 approximately and much better linearity in the driver stage if you just remove the 1K resistor. The minimum load that should be reflected to the plate of a 2A3 should be 2.5K.

Given the low source impedance of the driving 2A3 the IT should be very well behaved without any secondary load resistor, and 1K certainly isn't the right value.

Note that a 1K grid stopper installed right at the socket's grid pin is a good idea for both stages, this reduces the likelihood of oscillation, and EMI pickup.

A hand sketch of your schematic shot with a digital camera would be much better than nothing.

I take it you don't know how to simulate the amplifier. Spice models for the tubes are readily available online and a simple transformer model would suffice for your purposes. LTspice IV/Switchercad is free and available from Linear Tech here: Linear Technology - LTspice IV Downloads and Updates It runs fine in WinXP and also in Linux under Wine. There was a thread here a couple of years ago on how to use LTspice for tube simulations - I contributed extensively. (Still relevant even with the latest version of LTspice.)

The EML20 will not be compatible with your current IT, and the combined AC+DC plate load should be a minimum of 3X the tube's rp for reasonable linearity. (About 30K) Your current implementation isn't close to right if I understand you. I think the EML20 is only a reasonable choice for choke (or CCS) loading and would require a choke of 200H or more for really good LF performance.
 
Last edited:
The EML 20b -crazy priced.- was only as example to find out the gain it gives and compare to the 2A3..

The 1k is in series with the output 2A3 grid not in parallel with the IT.
To picture it...
So on the sec side of the IT:

The upper pin for the - 45 fixed bias and the
lower pin to the resistor to the grid of the output tube.

Drawn a schematic so you see...
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.