Ampeg GVT15H Owner's Manual download pdf (Page 7)

Guitar Amplifier GVT15H

™

Important Information about Tubes:

The Nature Of Tubes — Why (And When To) Replace Them:

Tubes are made up of a number of fragile mechanical components that are vacuum-sealed in a

glass envelope or bubble. The tube’s longevity is based on a number of factors which include

how hard and often the amplifier is played, vibration from the speakers, road travel, repeated

set up and tear down, etc.

Any time you notice a change in the amplifier’s performance, check the tubes first.

If it’s been awhile since the tubes were replaced and the amplifier lacks punch, fades in and

out, loses highs or lows or produces unusual sounds, the power tubes probably need to be

replaced. If the amplifier squeals, makes noise, loses gain, starts to hum, lacks “sensitivity”,

or feels as if it is working against you, the preamplifier tubes may need to be replaced.

The power tubes are subjected to considerably more stress than the preamplifier tubes.

Consequently, they almost always fail/degrade first. If deteriorating power tubes aren’t

replaced, they will ultimately fail. Depending on the failure mode, they may even cause severe

damage to the audio output transformer and/or other components in the amplifier. Replacing

the tubes before they fail completely has the potential to save time, money and other

unwanted trouble. Since power tubes work together in an amplifier, it is crucial that they

(if there is more than one) be replaced by a matched set. If you’re on the road a lot, we

recommend that you carry a spare matched set of replacement power tubes and their

associated driver tubes.

After turning off the power and disconnecting the amplifier from the power source, carefully

check the tubes (in bright light) for cracks or white spots inside the glass or any other

apparent damage. Then, with the power on, view the tubes in a dark room. Look for

preamplifier tubes that do not glow at all or power tubes that glow excessively red.

WARNING! Never turn on or use the amplifier without a load or speaker connected to the amplifier.

ALWAYS

use a good quality (non-shielded) speaker cable. Never use (shielded) instrument cable.

ALWAYS match the amplifier’s speaker output impedance to the impedance of the speaker

that is being used. Use only one type impedance output at a time. If more than one speaker is

connected at the same time, make sure they all have the same impedance rating. Never use

two or more cabinets with different impedance ratings. This will create an unbalanced load.

When using multiple speaker cabinets (with the same impedance rating), match the total load

impedance of the speaker cabinets to the speaker output of the amplifier.

SPEAKER CABINET IMPEDANCE divided by NUMBER OF CABINETS = TOTAL LOAD

16Ωcabswithparallelinputs/4cabs=4Ωload.Forthisapplicationuse4Ωoutputjacks.

16Ωcabs/2cabs=8Ωload.Forthisapplicationusethe8Ωspeakerjacks.

16Ωcab/1cab=16Ωload.Forthisapplicationusethe16Ωspeakerjack.

8Ωcabs/2cabs=4Ωload.Forthisapplicationusethe4Ωoutputjacks.

8Ωcab/1cab=8Ωload.Forthisapplicationusethe8Ωspeakerjack.

4Ωcab/1cab=4Ωload.Forthisapplicationusethe4Ωspeakerjack.

17. 16 OHMS:The16Ωspeakeroutputjackisdesignatedfor16Ωspeakercabinetsonly.

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Ampeg GVT15H Owner's Manual Page 7

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