Using a High Power OPT (400W) at half power

[TDS]

Is it okay to design a high power bass amp using a 400 watt OPT permanently at 200 watts? i.e. half power.

I am designing a big bass amp and want to use an OPT that I have at 200 watts instead of its design rating of 400 watts. The transformer is intended for 8 6550's and has a primary impedence of 1100 ohms with a 4 ohm secondary tapped at 2 ohms. An octet of 6550's seems complete overkill and I would prefer to use a quad of KT88's using the secondaries at 4 and 8 ohms respectively. The new reflected impedence of 2200 ohms seems quite good for 4 KT88's. I want to run them with about 730 volts on the plates and around 365 volts on the screens.

I know that in principle this should be okay as it is often done on guitar amps by pulling two tubes from a quad for half power and doubling the output impedence.

My question is: Is there anything flawed with running the transformer permanently in this mode and would there be any consequences I have not anticipated for the 4 KT88's?

TDS

[Don Moose]

Sure!

The bigger iron should even improve bass response.

[Satamax]

8EL34 should do it

[Enzo]

Or a WHOOOOOLE pile or EL84s....

Of course you cn use the high power tranny at low power. How would the tranny know the difference between a lower power circuit and a higher power curcuit turned down?

[TDS]

Enzo, That is exactly what I am thinking. I'm just lloking to the wide experience of those on this group who may be able to point out something I have overlooked.

TDS

[Satamax]

TDS, i was saying 8 el 34, because it's about right where they'd be happy. But anything will do the job. You can stuff a pair of 6C33CB, a pair of 6550 in triode mode. 6 or 8 EL34, 6KT88, or even may be four would live happy with you 1100Ω Enzo, CF the reply above Making a fool of myself again.

[Steve Conner]

It should work just fine, but there are a few things you should be aware of.

First, I assume you're going to connect, for example, an 8 ohm speaker to the 4 ohm tap. This will give a suitable load impedance for a 200 watt set of output tubes.

But, the magnetizing inductance of the transformer will have been designed to give adequate bass response with a 400 watt tube set. This means that the inductance will roll off the bass a little earlier with the 200 watt tube set, because it can only drive half the current. I know people say that more iron will give deeper bass, but in this case, that is not true.

However, if it was a high quality OT, that shouldn't be a problem. In fact, it may even cause a bit of extra harmonic distortion and make the bass sound bigger and fatter.

Whoever suggested 6C33s, those have an extremely low impedance and would give you bass right down to bedrock :-D They're also not too expensive.

I see you're in the UK, so is it one of the gigantic Danbury Electronics OPTs that Maplin used to sell?

steve

[TDS]

Steve, This is just the sort of info I am looking for. The magnetising inductance you mentioned is the sort of thing I suspected with less turns of thicker wire etc.

I will be using an 8 ohm load, at least to start with, using the cab currently used for a homebrew 100 watt version of a 5F6A Bassman. The 180 litre ported cab, also homebrew, has a Celestion K18J 18" 300 watt driver (98dB/1m) and a switchable 6" closed back mid-range speaker on a 800hz 1st order crossover. (Incidently, I go to Frank at Appian, up in your part of the world for all my speakers and any advice regarding speaker design). I would like to build a 4x10 bass cab later to use on top of the 18 cab. The two would then be able to run at 4 ohms.

The plan is to run it initially, or when using one cab at half-power, possibly switchable between 4 or 8 6550's/KT88's to suit the load used.

The OT is indeed a Dagnall, but it is current issue in the big VBA400 Marshall bass amp. I also have the matching toroidals for HT and Heater/bias. I intend to more or less use the VBA400 power amp and power supply circuit with a few of my own simplifications and preferences, combined with the tweed bassman type front end for the pre-amp.

At this moment I am designing a turret board for it. My next problem will be to find somebody to make a chassis for me, preferably in aluminium. Do you know anybody here in the UK who could custom make one?

[Steve Conner]

Hi TDS,

Sounds like the bass rig from heaven! If you're a bassist that is, hell if you're the sound guy or roadie

I'm currently working on something similar, but a bit smaller, using the OPT from a Fender 300PS that I bought from an Ebay seller in the states.

I'm having trouble with the chassis too, it needs to be seriously hefty to support so much iron! Even with an undersized PT, I'm looking at about 25lbs of transformers. I have a 4U aluminium 19" rack chassis that I bought from RS for another project, so I'm going to try and fit it in that, adding a load of bracing for extra strength. Nevertheless I'm afraid the rack mounting ears will just get ripped clean off.

I've built my own chassis from scratch before, because I don't know anyone who will do it for a good price. But the results weren't really up to a standard that I'd be happy selling to anyone.

steve

[Bruce / Mission Amps]

But, the magnetizing inductance of the transformer will have been designed to give adequate bass response with a 400 watt tube set. This means that the inductance will roll off the bass a little earlier with the 200 watt tube set, because it can only drive half the current. I know people say that more iron will give deeper bass, but in this case, that is not true.


steve

So there is no misunderstanding, I am not a magnetics guy, this field is not my forte, and by no means is a challenge.
However, I'm always interested in learning something new....

Isn't the output wattage ratting more of an AC function rather then DC?
I mean modulating DC is kind of the definition of AC.
Assuming the DC idle current through the OT from half the power tubes is high enough to effect the inductance in some positive way, (this I'm not sure of either as I thought high magnetisim in an OT was not a good thing)... I would have thought this power/current vs bandwidth issue would be a non-issue while the AC/DC current vs inductance would be more of an issue in a choke or swinging choke rather then an AC coupling transformer.

[Steve Conner]

Well, DC has nothing to do with it. In a push-pull amp, the DC magnetic fields cancel out and the core never sees them.

Regardless, any transformer is also an inductor. (After all, it's some copper wire wrapped around an iron core.) If you measure any winding on a transformer with an inductance meter, and the other windings open-circuited, you'll read something. This is called the magnetizing inductance, and ideally it would be infinitely large, so as to draw no current.

In practice it's not. It appears as an inductor hooked between your two output tube plates, or you can think of it as a much smaller inductor shunted across the speaker: it's the same thing. However you choose to think of it, it forms a L-R high pass filter in conjunction with the plate resistance of the power tubes. Pulling two of the tubes in a quad doubles the plate resistance and moves the -3dB point of the filter up an octave.

Negative feedback around the power stage helps to mitigate this. But NFB stops being much good as the power stage nears clipping and the tubes run out of drive. When the tubes are clipped hard the loop gain is zero and the NFB does nothing.

Also the magnetizing inductance is non-linear, being smaller at high power levels as the core nears saturation. Of course it varies instantaneously because the core magnetizes and demagnetizes in time to the audio waveform, but on average, it gets smaller.

The overall effect is that the bass gets compressed and clipped at high power levels, more so as you pull more tubes.

[Dave H]

Bruce,

It’s the shunting effect of the primary inductance in parallel with the reflected primary impedance which causes the bass to roll off.

I think the OT designer will have chosen the primary inductance to give the desired low frequency response with a 4 ohm load on the 4 ohm tap. If you ‘mismatch’ the OT by connecting an 8 ohm load to the 4 ohm tap I’d expect the bass to start rolling off an octave higher than with 4 ohms on the 4 ohm tap because the reflected impedance has doubled but the inductance (No. of turns) is unchanged.

Dave H.

[bob p]

However you choose to think of it, it forms a L-R high pass filter in conjunction with the plate resistance of the power tubes. Pulling two of the tubes in a quad doubles the plate resistance and moves the -3dB point of the filter up an octave.

by L-R do you mean Linkwitz-Riley, as in 24dB/octave? if so, I understand the abbreviation but not how we're looking at a Linkwitz-Riley filter.

[Don Moose]

by L-R do you mean Linkwitz-Riley, as in 24dB/octave? if so, I understand the abbreviation but not how we're looking at a Linkwitz-Riley filter.

I'm 99% sure L-R here means Inductor/Resistor the way RC means Resistor/Capacitor in this usage.

Steve would have been specific if he meant something esoteric.

('scuse me for speaking in your place, Mr. Conner)

[TDS]

To add a bit more information about the OPT, I measured the inductance and got the following:

Primary plate to plate 12.5H (secondary open circuit)

Primary plate to plate 2.5H (4 ohm resistor on 4 ohm tap)
5.0H (8 ohm resistor on 4 ohm tap)
Primary plate to plate 6.0mH (4 ohm taps short circuited)

I was using a DVM with an inductance range. I expected to see much higher inductances than I found. Does this sound correct?