My HAZE 40

modulator · 2019-08-07T08:48:40+00:00

I have mentioned my Marshall Haze 40 in the “For Sale” section in the past. I bought this amplifier (Serial code I-2010-05-….-E) because I thought my M...

My HAZE 40

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guido, I wonder if a 1 Watt amp would be good - not enough headroom maybe. (OK, the speaker sensitivity measured at 1 Watt thing aside - different guitars, different environment, tone perception, etc. Hey, everybody keeps on telling everybody else 3 dB is almost not noticeable, but they also recommend a 97 db speaker over a 95 dB speaker for more volume and more headroom and less distortion...) Seems like one requires a bit of volume to have fun anyway, unless the amplifier design would allow the sweet tone at low volumes. Including cleans and overdrive. Looking for that one. 5 Watt can be pretty loud, yeah. You should really poke a dB meter at it, and report back with details such as speaker sensitivity, estimated (measured?) amp output to speaker, guitar pickups, and so on.

guidothepimmp

Ive never played or heard a 1w amp in the flesh.. so difficult to gauge. The smallest ive had was a 5watter. I really didnt get along with it. Had 1 decent sound.. but cranked.. nothing. I still dont know if it was the amp.. or if it was the 10inch celestion in the amp. I always meant to play it through a 12 incher but alas. Never got to it.
When my amp guy was explaining my amp to me, he mentioned the pots having a sweet spot as opposed to the volume needing to be a certain volume for the sweet spot. Each pot was slightly different in where that sweet spot was. Basically he would sweep the pot min through max and there was apoint where there was a noticeable tonal or volume swell. This is the case on bass.. treb.. mids.. gain etc. His suggestion was to find that sweet spot on all the pots, then to dial back the master volume accordingly. Sounded bloody great. Was also bloody loud. The attenuator made by JP does a good job of reducing the volume to reasonable level.

I got rid of all the small amps because headroom was an issue. My smallest valve amp is an orange terror. It has the the 15w and 7w setting. The vokume difference between the 2 settings is a few db. Noticeable.. but not significant in my opinion. Headroom and gain was more noticeable. I will have a look see at the speakers.

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I tested the amplifier with the single coil Strat-alike as well, lower output pickups, potentially brighter sounds. The lower output pickups do allow me to set the amp volume knob higher, and the sound is just what one would expect from single coils: Bright, twangy, with the “quack” in positions 2 and 4 crystal clear. The speaker is fine. Working at “small venue” volume levels, it is fun. The tone really is thin. Playing around with the treble controls on the amp and guitar richens up the tone, but I would say, thin. It needs body. Overdrive channel is fun, at higher volume, and the Normal channel is bright, some would say glassy clean. No fun when turned down to very low “bedroom” volume, though.

As for the darker Overdrive channel, with this guitar it is easy to partly compensate by switching to the bridge pickup. It does not solve the issue, because one sometimes require the use of the neck pickup. And, for the first time ever, I experienced that sweet, syrup, smooth overdrive tones I could never get.

An aside: My Ibanez has the IBZ INF 1 and 2 pickups, I have not had it open to measure resistance, but it seems like they might be around 8.4 kOhm and 16 kOhm. The Strat-alike measures out at around 3.6 kOhm – 3.7 kOhm. I have for years been tempted to find Seymour Duncan Alnico Pro II humbuckers and single coil for the Ibanez, believing that the lower output pickups will lead to sweeter tone. With DC resistance of 8.8 kOhm (bridge), 6.1 kOhm (single coil) and 7.5 kOhm (neck), it is still higher than the Strat-alike, but it should help? Your opinions? The pickup upgrade will also include log pots and treble bleed and tone cap replacement. This might well result in that elusive syrup smooth sweet overdrive tones I cannot seem to get with it. The impedance mismatch in coil-split humbucker with middle single-coil settings might be a hassle? The middle pickup will dominate the position 2 and 4 settings? Might be interesting.

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I tried some amp setting alternatives to see if I can get the amp to sound better, such as stuff I never use - bright switch, boost switch. The amp is too loud, so I never use the Boost. The amp is generally too bright on the clean channel, so, the Bright never sees use. But, the bright might add some sparkle to the overdrive channel? As I plugged in, I noticed a lack of volume. I had to set the guitar and amplifier volumes up by three to four notches to get my normal volume. With the boost and bright switched in, the amp sounded wonderful. Just what I need, and, at a useful volume. But, the sudden low volume? I dragged the amp over, yes, all the tubes were glowing like they should. So the problem had to be on the guitar side – I recently re-soldered the angle jack after I pulled off the hot wire. Wiggling the cable at the angle plug suddenly released all the pent-up volume. It seems like a dry joint, wiggling the cable results in volume up and down. This implies that I can fit a signal attenuator between the guitar and amplifier, opening up the guitar volume pot, as well as the amplifier volume and gain pots, with the thicker sound from engaging the boost switches, and still play at bedroom volume.

Now, you electrical engineers, what happened here? I have continuity, but a weak signal, without loss of tone? I measured the cable, I could not induce any resistance variations. I could not measure any capacitance variations. Yet, I was losing signal strength.

I can mention here, the copper cable is oxidised, and experience shows solder does not want to stick to dirty copper. Am I making up a resistor/capacitor with this joint, while wiggling it will abrade oxidation off the contact point with the jack, and thus give clean signal when contacting there? And while you are at it, design me an attenuator box, one which will allow me to adjust guitar signal to the amp, without colouring the tone.

guidothepimmp

I think Klaasvakie is busy with an attenuator. And yes, he is an electrical engineer ?

Yeti

So the easiest way to attenuate the guitar signal is just with the volume pot on the guitar. If you want even more then a volume pedal will do the job. The problem is that you won't be hitting your pre-amp tubes very hard so they won't get any natural break up... also, the effect will be destroyed if you use any boost or OD pedals (although I suppose you could put the volume pedal after them if you want to). The other issue is that your signal is so low that you will be very susceptible to noise.

Klaasvakie's attenuator fits between the amp and the speaker. If you make this resistive then it's as simple as two high wattage resistors on a heatsink but you will lose some tone. The reactive versions aren't too complicated but you will need to wind your own coils.
I'm not familiar with your amp but if it doesn't have one already, you could always look at adding a master volume.

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Yeti
I have been playing with the guitar volume really low, about 2 - 3. Not what I need. I did try a simple pot in line, but the noise! I am sure attenuating the already weak guitar signal some more will add some noise, and I really do not want to buy more pedals. Maybe a graphic equalizer, yes.

The amplifier does have a master volume on the "Gain" channel, ideally needs one on the clean channel as well. Have been done, but I am not going to add stuff to this PCB thing.

I am thinking about a simple resistor attenuator for the speaker signal, even though it may "suck tone". The tone sucks at my playing volume anyway. Wind coils? No, man, just cut up an old speaker? My next post on this amp will mention the plan to add a bucking transformer to the mains supply, the amp is running a bit "hot" with 240 VAC.

I need to get this to somebody who can fit caps and swop resistors to modify the circuit slightly, I do not feel up to taking out the chassis to solder stuff out and in.

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This has been done a while (long) ago, only posting it now, with no further progress yet.

I adjusted the bias voltage to around 38.7 for Pin 1 on the CON3, and 38.2 mV on Pin 2 of CON3.

For fun, I measured the Voltages on Valve 5 as well, to chassis ground:
Pin 1 (Grid 3) 39.7 mV
Pin 2 (Heater) 3.4 V AC
Pin 3 (Plate) 440 V
Pin 4 (Grid 2) 434 V
Pin 5 (Grid 1) -40 V
Pin 6 Not Connected
Pin 7 (Heater/Cathode) 3.4 V AC
Pin 8 (Cathode) 39.7 mV
(Pin 1 is tied to Pin 8)

For Valve 4, the Pin1/Pin8 value is 38.9 mV. The difference in bias readings at the valves are due to component tolerances, I think? I doubt a 2% difference in bias voltage will matter much. Maybe just enough for some light assymetric amplification leading to pleasing, harmonic tones?

EL34 pins.

On the schematic Pin 7 is the Cathode, on the datasheet Pin 7 is heater? The schematic indicates Pin 2 and Pin 6 to be heater? My guess? They do not count Pin 6 (“not connected”) and thus tube pin 7 is schematic number “6” and tube pin 8 is schematic number “7”. Take care. Sorting this out took me some time.

Going from these measurements, how do I calculate plate dissipation? I think HT might be a bit high, my guestimate gives plate dissipation as 19 Watt. The heater voltages are almost up at maximum. Input to the power transformer was 237 VAC. I would suggest the circuit was designed for 220 VAC. Suggestions to easily drop the mains VAC to prolong valve life? At least, in this cabinet, there is a lot of space for a bucking transformer, I can easily fit one in the Mains line. Works for the 5F1 build-in-never-ending-progress.

May I add, recently I had 74 VAC supply in the house, at the same time the workshop was at 252 VAC supply. (I run from a 3-phase line in the street, not the same phases are tapped for the two buildings.) At 252 VAC input, I would guess I would not be doing my valve amps any favours.

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I cut off the top of the rear panel to open up the back, and started using a bucking transformer in the mains supply to lower mains feed to 222 VAC.

The “open back” sounds better I would say. The lower supply voltage does help to smooth out the amp as well.

With the 222 VAC mains supply the bias was now cool at 32 mV, and I re-set that up to 38.9 mV before measuring the voltages on the valves. While doing the measurements, I adjusted the bias again, to the values posted for the valve pins.

Valve 4 (EL34):
Pin 1 (Grid 3) 38.0 mV
Pin 2 (Heater) 3.17 V AC
Pin 3 (Plate) 413 V
Pin 4 (Grid 2) 407 V
Pin 5 (Grid 1) -37 V
Pin 6 Not Connected
Pin 7 (Heater/Cathode) 3.15 V AC
Pin 8 (Cathode) 38.0 mV

Valve 5 (EL34) Pin 1/8 38.7 mV

I am happier with the filament voltages now, but would still prefer a 215 V power supply.

I have also obtained JJ 12AY7 and JJ 5751valves to try in the pre-amp, and a JJ ECC82 (12AU7) to try in the phase inverter, to see if the tone smooths out some more.

Referring to the Celestion A Type frequency response graph, one could be fooled into thinking this speaker will remedy “Ice-Pick”. No, push the treble and volume a bit, and a 9 inch sharpened spike assaults your ears.

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I have since moved to a new town, where I have a reasonably constant 230 VAC supply, that is, when the National Supplier allows us to have it.

I checked bias again, it was a bit low. Adjusted up to 39.5 mV. The filament voltage is 6.6 V.
I need to get a 220 VAC supply.

Looking over all my research, I decided to risk the “lift C73” recommendation. It is the simplest one, no soldering new components. I can confirm that it will brighten up the Gain channel a lot. Not yet as bright as the Normal channel, but it is much better.

Thinking about the gain side of things, the “Boost” switch should add a lot of gain, so I tried that again. Oh, yes. This is good.

I am not convinced I need more brightness out of the Gain channel, maybe a bit less out of the Normal channel. The Boost funcion gives sufficient gain for my low volume playing, and I am sure, if I can open up the Volume to more than the bit I do, it will be very good.

The thing is still way too loud, but at least now, with C73 snipped, using the Boost on the Gain channel, and using the Master Volume to keep my ears on my head, I can get some fun sounds.

I have not yet made any progress with an attenuator, though.

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I hope this picture is readable.

The C73 capacitor is bottom right, at the Overdrive Master Volume. The placement seem to be meant to bleed higher frequencies to ground?

The other recommendation to add brightness to the overdrive channel, is to add more capacitance to C72, at the Overdrive pot, far left. This looks like a “bright” cap.

To increase gain, they say one should jumper R88, or make it a lower value.

Interesting is that the Overdrive Master Volume pot is ganged, the other half works as a sort of volume pot in front of the second gain stage V2A. The relay positions in this picture is set for the “normal” channel.

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L-PAD ATTENUATOR

After many years, I have finally put together a prototype L-Pad attenuator to test.

100 Watt resistors, 8 Ohm and 15 Ohm, to help tame my Marshall Haze 40 (16 Ohm speaker). The Haze 40 clean channel is loud, and sounds dull at the low volume I play at in my bedroom. It needs just a tad more gain to start sounding nice. The overdrive channel does have a master volume, no problem there. I play clean-ish, undistorted, mostly.
The options are an attenuator, power scaling the power amp, master volume. I am not sure what reducing the plate (and bias) voltage will do to EL34 tone, but, it is an invasive procedure. I do see the Studio Vintage 20 uses two EL34 valves at rather low plate voltage, cathode biased. Apparently sounds OK? Fitting a master volume is another invasive procedure, but probably the best option – I do not plan to go for power valve overdrive.
After reading all sorts of dire warnings on the internet, I made up this big heat sink. Uhm? I played for a while, at various too-loud-for-bedroom volumes, clean channel, drive channel, full gain, and so forth, and at low volume as well. The resistors do not really heat up much. So much for that, I can dispense with most of that heatsink. At the volume I am playing at, I am probably dissipating a Watt or so.
I cannot say it sucks tone too much, and it reduces output power just enough to enable me to open up the clean channel gain past the point where the tone becomes acceptable. I think more attenuation will work well. Some internet posters claim not having tone suck at large attenuation levels. At low volumes. The option of adding a bypass capacitor to restore lack of highs does exist.
The way forward might be building that reactive attenuator. I should look into a post phase inverter master volume. I should look into power scaling. I have no plans to plug the output into a re-amper.
The ideal option is to sell this amplifier, and maybe get something “more useful”. Only problem with that idea, is, these amplifiers have a very bad reputation, and are effectively worthless. I guess a set of new valves will cost more than what I might get for the amplifier if I try to sell it. So, hacking into it should not really reduce the value…