TAD 18-watt TMB – Reducing Hiss

[BenTobith]

I recently completed a build based on the TAD 18-watt TMB kit. (See schematic below.)

Deviations from the schematic:

• 470R / 3W screen grid resistors (in place of 100R / 2W)

• 150R / 5W cathode bias resistor (in place of 120R / 5W)

The above changes were in response to the amp initially running the EL84s wildly hot. Not quite red plating, but sizzling. In the current state, with the above resistor values, the plate voltage on the EL84s is ~354 VDC. The bias readings were an average of 42 mA, which seems to be around 125% of their plate dissipation. The power tubes are now running okay, no visual or other indicators otherwise. But of course, they are still running hot. (That might just be how this amp is supposed to be, I don’t know. But it’s worth noting our house notoriously has high wall voltage.)

The only other deviations from the TAD kit were as follows:

• Added a 47 pF cap across the two plate load resistors for the Phase Inverter

• Used a ground buss bar on the turret board instead of two buss bars across the back of pots

• Heater CT is “elevated” to cathode bypass cap of output tubes

• Mostly using metal film resistors (1 watt) for any resistors 100k or above and not in power section, with a few carbon film for the lower resistance values

• Using shielded cable (Mogami W2333) from the two inputs to grid, as well from the Volume pot (TMB channel) to V3 (cathode follower)

In its current state, the amp really does sound great, especially when you crank it. No hum or buzz, so I think grounding seems okay. TMB channel has a LOT of gain, especially on High input.

Potential issues/concerns:

• There’s some crosstalk between the channels, but I think that’s a “thing” with these?

• There’s a hiss from the speaker (whether a guitar is plugged in or not), made louder by turning up the TMB channel pots, especially treble and volume. Both High and Low inputs.

Everything about the amp when turned up and cooking I love. However, I do also like to use the Low inputs at lower volumes because I think the clean tone is fantastic. However, the amount of hiss you can get is a bit annoying. If I turn all of the pots down on the channel I’m not using, it can help, but the TMB channel is always hissy no matter what (that seems to be the main “source” of the hiss).

I’d love to find a way to reduce this hiss if possible, without dramatically killing the soul of the amp. Thoughts?

EDIT: I guess I should note I’m currently using long plate Mullards in all three preamp slots, a pair of Amperex EL84s, and a Mullard 6CA4 rectifier.

EF78575B-4702-47D9-8710-5662FEF84D6B.jpeg

[gktamps]

How are you hearing crosstalk? Are you plugging in two guitars, one to each channel?
Did you install a MV?
Have you swapped tubes or tried other tubes?
What is your bias now?
What is your wall voltage?
Using MF plate resistors helps with hiss, which apparently you did.

[BenTobith]

How are you hearing crosstalk? Are you plugging in two guitars, one to each channel?
Did you install a MV?
Have you swapped tubes or tried other tubes?
What is your bias now?
What is your wall voltage?
Using MF plate resistors helps with hiss, which apparently you did.

Hi, thank you for the response.

By crosstalk, I guess I mean that if I’m plugged into and using the first “channel,” I can hear more hiss/noise if the pots for the second TMB channel are turned up. When they are lowered, the overall noise floor of the amp is reduced. So there seems to be crosstalk in that context?

No Master Volume. That was one of the reasons I used the TAD kit as a starting point, I didn’t want MV.

I haven’t swapped too many tubes, but these are all “good” as I’ve tested them before.

Bias with the current setup is around 42 mA (41mA on one, 43mA on the other).

Wall voltage is usually around 125 VAC.

[pdf64]

What’s does the heater voltage measure? It’s a good habit to check the mains voltage anytime secondary etc voltage are measured. As that sets the context, everything else will follow suit, to some degree.

Crosstalk applies to signal. Each channel generates its own noise, presumably?

Aesthetically you may not want a master volume, but the reality looks to be that on the TMB channel, there’s a high gain / signal level going into the LTP and on to the output valves. Hence a potential divider between the tone stack and LTP may be beneficial in bringing that under control.

[gktamps]

Hi,

pdf64 beat me to a couple of your responses to my question, one of which was clarity on what you meant by "crosstalk" which he has explained is related to signal, not tube noise. However, when you turn up one unused channel while using the other, you will get additional hiss/noise. The problem is you have several issues that need to be normalized before you worry about the hiss.

First, please be very careful as you probe around your amp. Building an amp is one thing - dangerous enough - but when you start taking detailed measurements you need to be cautious about stored capacitor voltage, slipping probes, line voltages, etc. You may already know this, but since you don't have many posts and haven't listed your experience, it is reasonable for me to caution you.

The Marshall 18 TMB isn't really considered a "high-gain" amp, in that it doesn't have several gain stages, but it does have more gain hitting the PI as you've been advised, and therefore more hiss than the standard tremolo model. As pdf64 stated, the MV is a useful tool for both controlling hiss and permitting lower volume breakup. If you simply want really clean sounds you'll need to adjust the gain profile on the TMB channel, assuming there are no issues with the plate resistors or voltages.

Also good advice to check heater voltage - noise will be increased with high heater voltage.

Your wall voltage is quite high. You may need to build a bucking transformer to drop 5 volts or so if this is a normal voltage level. Your heater voltage will otherwise always be high, as will your B+.

You said your bias is about 42mA with Vp at 345VDC. You are WAY above the 100% setting for those tubes; the calculator shows you should be at 34.8mA maximum, or 33mA for 95% of maximum. That alone can cause increased noise and hiss - all of your preamp voltages may be running high as well.

You may need 160-180R cathode resistor to tame the bias, but you still need to measure all of the voltages in the amp after getting the bias under control. Remember your Vp will increase when you lower the bias current, so you'll need to measure all voltages and record them so you have useful data points.