Lifco model 630 speaker impedance and bias

[DeepBias]

Hello everyone! I’ve got some questions that I’d love some insights on, if anyone can help! 

I own a Lifco Regal model 600 amp for years, the head version without the matching cab. There I found the schematic online :

https://web.ncf.ca/ac151/lifco630.pdf

First concern is about speaker impedance, for an OT at 43:1 turn ratio wouldn't it be better a 4 ohm speaker instead of 8 ohm as mentionned on that schematic.

i also checked the bias using the primary OT resistance method :

tube 1
Primary OT resistance : 125,6 ohm
Voltage drop across promary OT : 3,62V
Plate current : 28,8 ma
Plate voltage across plate/cathode : 295,6V
Power dissipation : 8,57W

tube 2
Primary OT resistance : 145,5 ohm
Voltage drop across promary OT : 4,38V
Plate current : 30,1 ma
Plate voltage across plate/cathode : 295,5V
Power dissipation : 8,86W

Voltage across the cathode resistor is 9,8V

Isn't that a bit too low.

It has a pair of EL84EH and a 143 ohm shared cathode resistor actually and I was thinking of replacing it with a 120 ohm.

Love to play that amp cranked all the time but think it could sound better.

I hope someone can shed some light on this for me. Thanks so much in advance for your time!

[astronomicum]

9.8V across your 143 ohm cathode resistor yields 9.8V/143ohms=0.069A or 69mA. 0.069A at 295V yields 0.069A*295V=20.4W for two tubes or 10.2W each. A little bit higher than your method but low based on the data from the schematic you posted. The example indicates a plate voltage of 365V and an idle current of 72mA (10.8V/150ohms) yielding a dissipation of 13.1W (93.5%) per tube. The higher voltage provides a higher bias current. You should look into your lower voltage before making any changes. Start with looking at your rectifier and determine if the voltage drop across it is within acceptable parameters.

[DeepBias]

Hi astronomicum and everyone,

Thank you very much for the suggestion, here's some more voltage readings :

PT secondary : 305-0-305 VAC (310-0-310 unloaded)
5Y3GT B+ voltage (at first filtering cap) : 319 VDC (loaded) and 403 VDC (unloaded) ... Yields 84 VDC drop
Filament winding : 7,07 VAC (loaded) (12% higher)
5V winding : 5,7 VAC (loaded) (14% higher)

Seems an acceptable voltage drop from the 5Y3GT ? from 305 VAC to 319 VDC reveals a multiplicator of ~1.05

Based on the filament and 5V windings voltages readings I think that by hooking the amp on 115 VAC, they would be in the right spot, but it will also give a more lower B+ voltage.

The amp is drawing 0,612 A at primary PT (wall voltage ~124 VAC)

Lifco (Pepco) have produced lots of 630 models in various combinations (head, combo, 1 or 2 speakers), seems the PT secondary voltages on mine are low based on the schematic provided, but to me (and based on the pictures I can see online) the PT seems original. My 630 is actually inside a homemade head cabinet, I suspect the chassis is originally coming from a combo and the guy was tired of carrying it so he decided to put it inside a homemade head in order to lighten and make it more transportable.

Now about the speaker impedance, I can confirm that Lifco was delivering their combos with speaker(s) at 8 ohm total impedance, maybe they though more about sound instead of theory. I injected a small AC voltage at OT primary and based on the secondary voltage I can confirm the 43:1 ratio (mine is 44:1)

I used the OT resistance method to check the bias for a more accurate reading, excluding the screen current which is included by using the cathode resistor method.

Now I'm gonna try with a different 5Y3GT to see if I can get a little higher B+ and I'm not sure about replacing the cathode resistor, I heard that lower bias is better for an amp that will be played cranked, in combination with the mismatched speaker impedance, right ?

Any additional comments or suggestions will be greatly appreciated!

[HotBluePlates]

Hello everyone! I’ve got some questions that I’d love some insights on, if anyone can help! 

I own a Lifco Regal model 600 amp for years, the head version without the matching cab. There I found the schematic online :

https://web.ncf.ca/ac151/lifco630.pdf
tube 1
...
Power dissipation : 8,57W

tube 2
...
Power dissipation : 8,86W

Voltage across the cathode resistor is 9,8V

Isn't that a bit too low.

I see at least 500 posts complaining about overheating-EL84s for every 1 post where someone wonders if their EL84s are idling too cold.  Based on that alone, I'd tell you to leave your amp as-is.

You observed 9,8v of bias in your amp, and the schematic says you "should" have 10,8v.  But notice some things:
   - Your tubes are idling at <9 watts each.
   - If you raised the bias voltage to 10,8v then your tubes would idle even cooler.

   - The above observations mean that you might just have a cool-running pair of EL84s, or that their Transconductance is extra-high (which tames plate-current with less grid-voltage/bias-voltage).

Maybe you meant that the idle plate dissipation was "too low."

   - These particular tubes could "a cool-running pair of EL84s" in which case changing the cathode resistor to heat them up will make the next set of EL84s over-hot.

   - These particular EL84s could have weak/worn-out cathodes, in which case getting plate current up to 12w dissipation will overheat the next pair of EL84s.

No... I think I'd advise you leave the cathode resistor alone, especially since you're on the "safe side" of any doubt.

PT secondary : 305-0-305 VAC (310-0-310 unloaded)
5Y3GT B+ voltage (at first filtering cap) : 319 VDC (loaded) and 403 VDC (unloaded) ... Yields 84 VDC drop
...
Seems an acceptable voltage drop from the 5Y3GT ? from 305 VAC to 319 VDC reveals a multiplicator of ~1.05
...

The multiplier is "Observation" and not "A Rule."

The DC Volts output of the Rectifier is determined by:
  - PT AC Volts
  - PT Winding Resistance
  - Rectifier Internal Resistance
  - Filter Capacitance
  - Amplifier Direct-Current draw from Filter Cap(s)

  - Plus a few more things when thinking of an always-applicable mental model.

If you change the filter caps (they could be worn) or otherwise increase µFs, then the DC volts will go up.  And maybe the guy who drew that schematic was using a Russian "5Y3" that doesn't drop voltage like a real 5Y3 (and instead like the low-voltage-drop 5V4 instead).

You also noted heater voltages seemed correct with 115vac, so what if the other guy used 120v+?

Now about the speaker impedance, I can confirm that Lifco was delivering their combos with speaker(s) at 8 ohm total impedance, maybe they though more about sound instead of theory. I injected a small AC voltage at OT primary and based on the secondary voltage I can confirm the 43:1 ratio (mine is 44:1)

44:1 and an 8Ω speaker implies about 15.5kΩ primary.

That's a perfectly reasonable plate load for EL84s, especially if they are intended to remain in Class A and B+ voltage might drift towards the high side.

The EL84 can pull its place down to about 60v (top of Page 4 here).
You have 296v plate-to-cathode according to your 1st post, so the EL84 plate might swing 296v - 60v = 236v Peak.
Your tubes are idling at about 30mA plate current; the limit of Class A would be Peak plate current change of +30mA and -30mA.

236v Peak / 30mA Peak = 7867Ω to one output tube
OT plate-to-plate Impedance = 2 x the load to one side = 2 x 7867Ω = 15,7kΩ ---> 14,5kΩ is "close enough"

[DeepBias]

Hey HotBluePlates !

Thanks so much for setting the scene so nicely, I'm 100% with you ... I'm not worried about the voltages shown on the schematic compared to what I got in my amp, and that's a good advice to swap some other rectifier / power tubes and see what happen.

The only changes I did until now are to replace the filtering capacitors and made it 3 prongs. No leak across bypass caps and all resistors are in the good range.

Yes that's correct, I was concerned about the cold bias versus the available voltage for the power tubes in that amplifier, but after reading your post I think for now I'll put that aside, the amp is loud enough.

I'm doing some tests right now, I want to remove the tremolo and use that 6AV6 as an additionnal preamp stage. The amplifier is not original, inside an homemade head. I have another one which is all original, exactly with the same chassis but inside its original head (with the cream bar on the front) ... so I did a little 6AV6 external stage on the breadboard then I desoldered C13 for hooking my new stage there on V2 pin 1... That was a revelation ! Just like a cranked JCM800.

Now I'm going to remove all the tremolo parts, not a problem but where I'm not sure is about to disconnect the wire going to V3 cathode, is it that simple.

https://web.ncf.ca/ac151/lifco630.pdf

My plan is to add an additionnal but switchable stage to swap between the original input and the new added stage and then get the best of the 2 worlds, the clean is awesome on that amp. For now I'm gonnna use a 3PDT switch so I can ground the unused new stage, I tried to figure out a wiring diagram by using a DPDT but I can't see how to hook 2 différent input circuits with such a switch, well I can do that but the original network (R5/R26/C13) turn out into a 370K to ground resistor while I'm on the new stage position, not sure about how it could affect the input on the new stage, so the 3PDT is an easier solution.

Gonna keep coming back and let you know about the evolution, again thank so much for the good demonstration, this is time saving and always appreciated, everyone feel comfortable for any comment and suggestion.

[astronomicum]

Have you tried a different 5Y3GT like you indicated?

[DeepBias]

Have you tried a different 5Y3GT like you indicated?

Hey astronomicum, not yet but this is in the todo list for sure. I'm curious to see if I could get some more voltage, or only to check if that 5Y3GT is a little tired ... Will also try with other EL84, even with 6BQ5 or 6P14P. I'll be back soon with the results.

[AlNewman]

I don't think there's anything wrong with changing the cathode resistor to find a higher output wattage, except, perhaps the power transformer is already performing at it's peak load.  You may find by experimenting that you can't get more power at the plates, because the transformer can't keep up.  Meaning you sacrifice voltage for current, or vise versa. 

[DeepBias]

Hi AlNewman,

To have fun by learning and trying to tweak here and there is indeed not wrong at all, at least as long as that fun is not inside an original JTM50  I owned one being younger and sold it for $300 

I dunno about the PT peak load, what I can say is that it's not getting hot at all after playing with it for a long time, even not so warm. I will start by swapping some other tubes in there, then for sure after the additionnal stage added I'm gonna check if I can (or should) do something with the bias, I heard somewhere that by running the power tubes not too hot, they can last quite a long time. So if I replace the actual 143 ohm, for example with a 120 or 130 ohm cathode resistor and see a voltage drop, means that the PT can't hold more current .. right ?

Have a good evening and thanks very much for the explanations!

[HotBluePlates]

... if I replace the actual 143 ohm, for example with a 120 or 130 ohm cathode resistor and see a voltage drop, means that the PT can't hold more current .. right ?

Volts = Resistance x Current

The Volts across the Cathode Resistor is the tube's Bias.
For the tube to pass more Current, Volts must go Down.
But you are lowering Resistance to get there, which would tend to keep Volts the same.
You will find it takes a pretty large-% change of Resistance to move Volts, while Current is moving the opposite way.

I don't think you'll achieve much by biasing hotter, except that the output tubes will be slightly easier to push to distortion (though that will happen at the same loudness).

I have a set of EL84s in a 1961 AC30 right now.  That set tends to idle around 9 watts, but I'm sure not changing the original parts in this amp to get that one quad to idle hotter.  Instead, I'd just grab different EL84s.

[DeepBias]

I don't think you'll achieve much by biasing hotter, except that the output tubes will be slightly easier to push to distortion (though that will happen at the same loudness).

I have a set of EL84s in a 1961 AC30 right now.  That set tends to idle around 9 watts, but I'm sure not changing the original parts in this amp to get that one quad to idle hotter.  Instead, I'd just grab different EL84s.

Yes Sir, I already replied about the bias in post #4, it's very clear to me 

For now I'm wondering about the removal of the tremolo circuit, I'm quoting myself (see post #4 for details), but here is the main part of what I'm not sure about :

Now I'm going to remove all the tremolo parts, not a problem but where I'm not sure is about disconnecting the wire going to V3 cathode, is it that simple.

It would be helpful if someone could take a look at the schematic and confirm this.

Thanks as always for the great support and for sharing your expertise, really appreciated!

[passaloutre]

Yes just remove the wire between the two cathodes and you will eliminate the tremolo effect. Then you can use the extra triode somewhere else.

[HotBluePlates]

Now I'm going to remove all the tremolo parts, not a problem but where I'm not sure is about disconnecting the wire going to V3 cathode, is it that simple.

It would be helpful if someone could take a look at the schematic and confirm this.

Yes just remove the wire between the two cathodes and you will eliminate the tremolo effect. ...

I agree.  It really is that simple.

[DeepBias]

Yes just remove the wire between the two cathodes and you will eliminate the tremolo effect. Then you can use the extra triode somewhere else.

... I agree.  It really is that simple.

Thanks for confirming    I knew I was on the right track, but advice from more experienced people is always welcome to avoid screwing things up. 
 

[DeepBias]

Have you tried a different 5Y3GT like you indicated?

Hey astronomicum, not yet but this is in the todo list for sure. I'm curious to see if I could get some more voltage, or only to check if that 5Y3GT is a little tired ... Will also try with other EL84, even with 6BQ5 or 6P14P. I'll be back soon with the results.

I tried with 5Y3GT from different brands (6) and also with a couple of EL84/6BQ5/6P14P matched tubes, the voltages are about the same (~8V), so I will have to deal with it I think. Not that much concerned with the power transformer,

I even tried the amp connected to a bucking transformer (115V) and I cannot hear much of a change in the tone, maybe it sounds better with the volume at max, but for sure the heater & 5V taps are much closer to specs at 115V, and B+ is even lower, not sure if I'm gonna leave it like that or maybe decrease Rk at 120 ohm, I did test using a 820 ohm resistor in parallel with the 143 ohm inside the amp using alligator clips and again I'm not sure which I prefer.

Right now I'm working on adding a new preamp stage using the tremolo 6AV6, with this additionnal stage the amp sounds really good, never I would have imagined such a change, still working on the switching concept but I'm not that far from what I'm seeking.

Thanks again for the good advice, maybe some sound clips when I will be done with that switchable preamp stage?