Buzz/Fizz from any overdrive

[Kodou]

Hi guys,

Last week I got a 1978 Hiwatt DR103 2 Input and a 1977 SE4129 Cab with Fane 122667 speakers.
When I tried it out before buying I only played it clean as I live in an apartment and it was 9pm. It sounded fantastic and really quiet so I went with it.
A few days later I cranked the normal and brill channels with the MV down pretty low (see above mention of being in an apartment) and noticed that there was a buzzing transistor radio like sound that was completely killing the good tone of the amp. I did some research and discovered the Fizz issue. I've considered modding it to the mid 70s spec (removing the bridging wire on v2 and soldering 2 resistors in) but thought that if I was planning on using pedals mostly then it won't be that much of an issue. That was until I put an early 80s Tube Screamer and a Jam Pedals Red Muck into it. Essentially I discovered that no matter what volume the channels and MV are on, any distortion sets off the transistor buzz, effectively ruining the pedal sound. It really does sound awful doing anything besides clean.

Is this what the fizz problem does? I thought it was just from getting the natural valve distortion, not pedals.
If it is the fizz issue then I will be modding it ASAP however I thought I'd check here to see what you guys thought.

I have dreamed of owning a Hiwatt Custom 100 for ages, so I really hope this is an easy fix. I'd hate to have bought a complete lemon

I should point out that the previous owner has owned it since 1990 and has maybe played it for 3 hours in all that time. It is in fantastic condition and looks all original inside.
The pre-amp tubes are the original Mullard however the power amp tubes are a mix bag. 1 Groove Tube, 1 National, 1 labelled EL34 Made in GBR and 1 blank.
Also one tube has lost it's guide key however I have marked it and made sure it is installed correctly.

[mhuss]

The fizz comes from blocking distortion in the second stage, and this happens because of both the signal level and the fact there is is little or no resistance in line when the input volumes are turned way up. Are you overdriving the input with the pedals, i.e., is the volume a lot louder with the pedal engaged? If so, the same thing will happen unless the input volume is set fairly low. If you adjust the pedal output control so the volume is the same with the pedal on or off, and the input volumes are less than half, there should be little or no fizz.

If you do have a fizzy two-input amp, I recommend adding the resistors, using the 'alternate mod' described on Hiwatt.org. This is easily reversed if you want to convert it back to stock.

Nice looking amp.

--mark

[Kodou]

After speaking with the previous owner we think it may be the speakers. The cab has been in storage for 12 years so the speakers may be stuffed. I'm going to try another cab tomorrow so I'll report back.

[Dr.HI-TONE]

if it were the speakers, it would be noticeable at low volume.

[Kodou]

I took it over to a friends yesterday and tried the cab with his head and my head with his cab. Both times it sounded great.

I then plugged the DR103 into the Hiwatt cab and put the normal channel on 9 and the MV on 2.5ish. Hit a chord and it sounded amazing, everything a cranked Hiwatt should sound like.

That left me a bit mystified as I will swear it sounded awful when I tried it last.
We put a few pedals through it and each one sounded great.

It then occured to me that, when in my apartment which is on the small side, I play with the MV probably just under 1. I turned it down to there and all of a sudden the amp and pedals sounded like a transistor radio. It seems that it just hates being played that low. I will do the De-Fizz mod and maybe that will clear the problem up however it is good to know that at real band levels, the amp sounds amazing.

I'll report back if anything changes as I have rehearsal this Thursday which will be the first proper play of the amp in a live situation.

Possibly a good attenuator will help with playing at apartment levels.