An intriguing 6S45P direct coupled 300B amplifier

[Antonio Tucci]

I’d like to submit and discuss the schematic of an integrated amplifier (direct connected to a CD-player … without pre).


We (no-profit association – Agorà) would like to realize this project together with the child that take part to the “Corso di Elettronica” (electronic course, promoted by our association).

Since I’m not an expert in Electronic, I’d be very grateful if somebody could help us in this adventure.



Points to be defined:

Current setting of 6S45P and bias of 300B:
1) 15-20 mA …with “combined” anode load (H + R; i.e. H=50H/400DCR+ R=3.000 ohm).
2) 35 mA … with “pure” choke anode load (i.e. 170H/2000DCR).

6S45P-Bias:
1) vacuum tube diode on the cathode (which one?).
2) battery on the cathode.
3) RC on the cathode.
4) RC on the cathode (with R being a potentiometer … in order to match the two 6S45P).

Input transformer:
1) Sowther 3575 in configuration 1:1
2) Sowther 8920 in configuration 1:1 having the two secondary in parallel.
3) Sowther 8920 in configuration 1:2 … with a less power input-driver tube (i.e. ECC99).

Power supply:
1) L2 and L4 --> 30H/100mA/190DCR
2) V ?
3) C1=160 uF, C3=80 uF, C5=1 uF

[Antonio Tucci]

...

[analog_sa]

Nice looking project!

Current setting of 6S45P and bias of 300B:

6S45

I don't quite understand the need for a combined choke. If it's purely dc considerations it is possible to adjust the anode voltage as you have a separate PS. For 6S45 even 50H would suffice IMO.
So my choice would be 50H @15mA.

6S45 bias

Vacuum tube? Don't think any will give you such a low bias at 15mA, let alone 35mA. If you still think you must use it try several 80 in parallel (2-4?)
IME rechargable batteries work well with the 6S45. Just don't ask what type as they all sound annoyingly different (as capacitors) in this position. As you have an input transformer it may be a much better sonic solution to connect the battery in series with the transformer windings.

regards

[ThorstenL]

Konnichiwa,

Originally posted by Antonio Tucci
Current setting of 6S45P
1) 15-20 mA …with “combined” anode load (H + R; i.e. H=50H/400DCR+ R=3.000 ohm).
2) 35 mA … with “pure” choke anode load (i.e. 170H/2000DCR).

From experience with the 6S45-PE, you may find neither chosen operating point too good sounding. I would recommend 170V/10mA as a starting point for optimisation.

Originally posted by Antonio Tucci
and bias of 300B:

In my experience and if the desire is not to exploit the maximum power of the 300B, try 350V/60mA/5K load with 74V Bias. Then consider the use of one or two LNR Diodes, carefully matched to the circuit (that requires access to serious Audio Analyser and the skills to use it, plus a box full of different diodes). If you are not certain how select the correct LNR diode, leave them out as the wrong diode will make things worse rather than better.

Originally posted by Antonio Tucci
6S45P-Bias:
1) vacuum tube diode on the cathode (which one?).
2) battery on the cathode.
3) RC on the cathode.
4) RC on the cathode (with R being a potentiometer … in order to match the two 6S45P).

May I suggest a better solution entierly? Battery on Grid. As you already have intended an input transformer (for an integrated amplifier unit a Volume Control Transformer may be a better choice) just place the battery in the ground lead of the Transformer making sure to bypass the battery with a capacitor.

The 6S45 choke loaded will allow the required 74V Peak at around 0.5% 2nd harmonics, so the driver stage will be very linear, using a LNR Diode will not likely make things significantly better.

Then to correctly Bias the 300B add a small adjustable resistor in series with the 6S45-PE Anode Load choke which allows the 300B Bias to be trimmed.

The suggested operating points will allow around 6 - 7 Watt RMS @ 5% THD (mostly 2nd Harmonics) without using LNR Diodes and will require around 1.3V RMS for full output with a 1:1 Input connection.

Sayonara

[Antonio Tucci]

KYW,
thank you for your replay.

Specific questions:

1) 6S45P Operating point.
OK for your suggestion. However ... how to obtain the bias for the 300B with only 10 mA ? We should have to add a 17Kohm R (better ... 17.000-400 ohm of the choke) in series with the anode choke.
Can the high value of R represent a problem ... or make worse the operating condition of 6S45P ?


2) Battery bypassed by a capacitor on the ground lag of the input Transformer.

OK...but which V value and C value ? ... 2.4 V ? ... uF ?


3) LNR diode on the cathode of 300B. (what does LNR mean ?)

Why do you believe that the use of 6CG3 may represent a problem ? Since it has been largely experimentated and tested by Dennis Boyle ... why not to use it ?

Dennis Boyle told me that a mixed bias is better than a fixed bias. I do not know the reasons of such assertion. However, I believe that it must be important ... if he utilizes, in addition to the stacked supply, a LNR diode on the cathode.

Furthermore, eliminating the LNR diode ... what we need on the cathode of 300B ? ... nothing ? ...or a Capacitor ?

[Antonio Tucci]

For analog...

I don't quite understand the need for a combined choke. If it's purely dc considerations it is possible to adjust the anode voltage as you have a separate PS. For 6S45 even 50H would suffice IMO.

...The combined load (choke+R) is necessary to obtain the bias for the 300B.
Alternatively, with 15 mA ... how to obtain 70-75 for the bias ?

[jlsem]

Since I’m not an expert in Electronic, I’d be very grateful if somebody could help us in this adventure.

Are you sure that a 5AR4 is the correct rectifier for the 6C45 B+? I don't see how you can draw the B+ from its center-tapped filament.

John

[jlsem]

Vacuum tube? Don't think any will give you such a low bias at 15mA, let alone 35mA.

I believe the 6CG3 will work in this position. Dennis Boyle's Chimera Labs website shows lower distortion figures when using (low internal resistance) damper diodes for setting bias.

John

[Antonio Tucci]

Jlsem,
I do not understand the problem with 5AR4 ...

Are you sure that a 5AR4 is the correct rectifier for the 6C45 B+? I don't see how you can draw the B+ from its center-tapped filament.

Please, may you better explain the problem ?

[Yugo]

Originally posted by jlsem


Are you sure that a 5AR4 is the correct rectifier for the 6C45 B+? I don't see how you can draw the B+ from its center-tapped filament.

John

jlsem,
you can take B+ ad libitum from either side of the DH rctf. eliminating the pot (or two matched R's), it does not change anything except you get better and more equall and linear exploatation of the rctf. cathode, compairing with the case when it's taken from one side.Or better try a separate,good, with e- static shield,bifillary wound heater T/X!!

Regards,
Yugovitz