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ECC83S/12AX7 Omitting grid leak, input impedance and preamps

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#25
Well I wish I could answer these questions. I am fairly inexperienced yet. I've inherited this DIY amplifier (literally, father in law passed away) without explaining me the topology and design choices. There was no circuit diagram, I've drawn it myself. This amp is basically the very first thing I worked on and it has kick started my interest in electronics. I have no prior electronics experience.

The amplifier was built for automotive use. It had 12v PSU, which I have already replaced with a proper transformer from toroidy. It was designed to take in the balanced input, yet it was never used with such. The pins are shorted and take in an unbalanced input now. I do wonder though what is the benefit of using such amplifier with an unbalanced input, twice the gain..?

From what I understand, it internally works as a push-pull balanced amp - 2x ecc83s (double triodes - total 4) on line stage and 4x 6l6gc on power stage. I believe the input transformer are used for the sake of using balanced input plus to remove dc coupling.
 
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#26
operatorius said:
So now I have two problems 😀 !
1. Buy new transformers
2. Adapt the existing circuit to match the secondaries nominal and/or choose an appropriate transformer for this case.

If I have understood you correctly, it is pretty tricky to calculate the load the transformer's secondaries see ?
Click to expand...
New 10K:10K transformers is the simplest and most effective but not the cheapest solution. Making sure the transformer is loaded properly is simply a matter of placing a 10K resistor across the secondary. OEP do some reasonably priced 10K:10K transformers that perform surprisingly well. I have attached the results of some tests I did on them.

I think you can forget about the grid stoppers. They are not really necessary with an ECC83 and they will only curtail the HF response due to the Miller effect. Personally I would short them out and add a 1Meg from the grid to ground ust to ensure the grid has a dc ground reference.

Note that the input transformer does two things; it allows a balanced input to be used and it acts as a phase splitter (a technique not used much if at all these days).

Cheers

Ian
 

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#27
ruffrecords said:
New 10K:10K transformers is the simplest and most effective but not the cheapest solution. Making sure the transformer is loaded properly is simply a matter of placing a 10K resistor across the secondary. OEP do some reasonably priced 10K:10K transformers that perform surprisingly well. I have attached the results of some tests I did on them.

I think you can forget about the grid stoppers. They are not really necessary with an ECC83 and they will only curtail the HF response due to the Miller effect. Personally I would short them out and add a 1Meg from the grid to ground ust to ensure the grid has a dc ground reference.

Note that the input transformer does two things; it allows a balanced input to be used and it acts as a phase splitter (a technique not used much if at all these days).

Cheers

Ian
Click to expand...
Well most of the trafos are located either in USA or UK. Shipping to EU would impose taxes making them really expensive. Found some discounted Sowters on don-audio located in Germany which would amount to ~180 euros >.<

Just out of curiosity, what do you mean by "most effective" solution? I reckon there are more ways to split the phases?
 
#28
operatorius said:
Well most of the trafos are located either in USA or UK. Shipping to EU would impose taxes making them really expensive. Found some discounted Sowters on don-audio located in Germany which would amount to ~180 euros >.<
Click to expand...

Sounds like the EU playing silly buggers as usual. I have no trouble importing stuff into the UK from the EU without extra taxes or import duty. 180 Euros seems about right for a pair of Sowters. They are getting on for £80 in the UK. Surely you have RS in Germany? You should be able to get the OEP K30A06C from them for £40 + VAT.
operatorius said:
Just out of curiosity, what do you mean by "most effective" solution? I reckon there are more ways to split the phases?
Click to expand...
The are definitely alternative solutions. Whether they are any better is open to debate. Phase splitters are very common - you can make one from a one or two triodes, so one alternative would be to redesign the first stage of the line stage to be a regular unbalanced input phase splitter.

Cheers

Ian
 
#29
ruffrecords said:
Sounds like the EU playing silly buggers as usual. I have no trouble importing stuff into the UK from the EU without extra taxes or import duty. 180 Euros seems about right for a pair of Sowters. They are getting on for £80 in the UK. Surely you have RS in Germany? You should be able to get the OEP K30A06C from them for £40 + VAT.

The are definitely alternative solutions. Whether they are any better is open to debate. Phase splitters are very common - you can make one from a one or two triodes, so one alternative would be to redesign the first stage of the line stage to be a regular unbalanced input phase splitter.

Cheers

Ian
Click to expand...
Are there any topologies of phase splitters that I could look into? I am aware of the "concertina" using a triode.
At this stage, investing over 180e, I am even concidering building myself another amp instead. I've come to a realization, that push-pulls need to have matched tubes on power stage. As the phase inversion is done before the line stage, the double ecc83s triodes have to be ballanced as well. I cannot guarantee that this was taken into consideration when the amp was built...
 
#30
Autos are a very noisy environment electrically, with all kinds of RF interference to try to deal with. One side benefit of input transformers is that they're pretty good at filtering RF junk before it can get into the circuitry. Autos also have weird issues with grounding, and here again input transformers provide a unique solution (complete galvanic isolation if wanted).

But normal home use is less demanding, and normally doesn't need heroic efforts. A "perfect" input transformer could still be called an ideal, but perfection is reserved for $heaven$ and slightly better bench performance is possible without the input transformer. Many of the advantages of the transformer are available from an active differential input stage - in your case a CCS in the (common) cathode path of the input 12AX7 stages. Even this small effort is rarely needed in normal home use amplifiers - single-ended seems fine for almost all cases with proper safety ground (PE) management.

All good fortune,
Chris
 
#31
operatorius said:
Are there any topologies of phase splitters that I could look into? I am aware of the "concertina" using a triode.
At this stage, investing over 180e, I am even concidering building myself another amp instead. I've come to a realization, that push-pulls need to have matched tubes on power stage. As the phase inversion is done before the line stage, the double ecc83s triodes have to be ballanced as well. I cannot guarantee that this was taken into consideration when the amp was built...
Click to expand...
Check out Merlin Blencoe's we site. He has lots of good basic info:

http://www.valvewizard.co.uk/

Cheers

Ian
 
#32
#33
The input section offers very low impedance. Not usable to most signal sources. You will need a buffer or another preamp to match your source.
Do not try to place any volume control in your existing circuit. It will only degrade the quality of the sound.
Regards..
 
#37
Crazy thought I am sure but could one add a series resistor (with or without the existing grid stopper) in each grid leg and a pot between the legs right after (on the grid side) to form a two sided voltage divider essentially partially shorting the input signal between the phases?

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#38
operatorius said:
I think it has to do with the fact that at input there's a 600:600 ohm transformer that is unloaded. Even with loading, to ensure its normal operation, the max load should be 600 ohms thus the reflected on primaries would be 600 as well. Right?
Click to expand...
The input impedance of that stage will be the inductive reactance of the primary winding.

If you can find the inductance of the primary winding (H), you can calculate the inductive reactance (Xl) (wrt frequency) and that will be the input impedance (at that frequency).

Things change a bit when you load the secondary, as do other things, so you'd want to take a broad view on that.
 
#39
There is a limit on how much max the load impedance can be.
If you load the secondary with a 1 mega ohm resistor, does not mean the primary will have the same input impedance.
Then there is no meaning saying 600 ohm transformer. They will say only 1:! transformer.
Matching the impedance to 600 ohms is only for maximum power transfer, which is not applicable in this case.
We have a voltage transfer.. So the driver to the primary must be able to supply at least the drive voltage required for maximum power on the output tubes
And at this low impedance. As an example if 1 volt rms is required at the grid of the splitter to drive the output to maximum power, then the source driving the primary must be able to supply 1 volt plus losses and at a very low impedance (may be 5-10%) of the reflected impedance at the primary.
If each of the secondary load is 10 k,the the primary will be 5 k, then the source must have less than 250 ohms output impedance. Otherwise most of the signal will be lost.
Regards.
 
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#40
While this is true through the mid-band of a transformer's frequency range, it begins to fail as a model when approaching frequency extremes (in the audio band's 1:1000 range). A decently good model of a transformer, ignoring the weird dynamic stuff happening in a metallic core, is of a "perfect" idealized transformer surrounded by parasitic elements, usually simplified to lumped-sum elements external to the "perfect" internal transformer.

Unavoidable parasitics include a primary (magnetizing) inductance and capacitance in shunt and a primary leakage inductance and resistance in series, etc. The balance in the choice of these parasitics, which must be traded off amongst each other, give a transformer its specified impedance. Operation too far outside of design goals has penalties in performance.

As folks have already said, a 600 Ohm (specified) transformer will have too little primary inductance to operate well at 10,000 Ohms, and the reverse is also true. A 10,000 Ohm (specified) transformer will have too much leakage inductance and primary and secondary resistance to operate well at 600 Ohms. Transformers are very particular critters.

All good fortune,
Chris
 
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